@article {3407, title = {A Case for Synthesis of Recursive Quantum Unitary Programs}, journal = {Proceedings of the ACM on Programming Languages}, volume = {8}, year = {2024}, month = {12/5/2023}, pages = {1759{\textendash}1788}, abstract = {

Quantum programs are notoriously difficult to code and verify due to unintuitive quantum knowledge associated with quantum programming. Automated tools relieving the tedium and errors associated with low-level quantum details would hence be highly desirable. In this paper, we initiate the study of program synthesis for quantum unitary programs that recursively define a family of unitary circuits for different input sizes, which are widely used in existing quantum programming languages. Specifically, we present QSynth, the first quantum program synthesis framework, including a new inductive quantum programming language, its specification, a sound logic for reasoning, and an encoding of the reasoning procedure into SMT instances. By leveraging existing SMT solvers, QSynth successfully synthesizes ten quantum unitary programs including quantum adder circuits, quantum eigenvalue inversion circuits and Quantum Fourier Transformation, which can be readily transpiled to executable programs on major quantum platforms, e.g., Q$\#$, IBM Qiskit, and AWS Braket.

}, issn = {2475-1421}, doi = {10.1145/3632901}, url = {https://arxiv.org/abs/2311.11503}, author = {Deng, Haowei and Tao, Runzhou and Peng, Yuxiang and Wu, Xiaodi} } @article {3506, title = {Clifford-Deformed Surface Codes}, journal = {PRX Quantum}, volume = {5}, year = {2024}, month = {3/19/2024}, abstract = {

Various realizations of Kitaev\&$\#$39;s surface code perform surprisingly well for biased Pauli noise. Attracted by these potential gains, we study the performance of Clifford-deformed surface codes (CDSCs) obtained from the surface code by applying single-qubit Clifford operators. We first analyze CDSCs on the 3\×3 square lattice and find that, depending on the noise bias, their logical error rates can differ by orders of magnitude. To explain the observed behavior, we introduce the effective distance d\′, which reduces to the standard distance for unbiased noise. To study CDSC performance in the thermodynamic limit, we focus on random CDSCs. Using the statistical mechanical mapping for quantum codes, we uncover a phase diagram that describes random CDSC families with 50\% threshold at infinite bias. In the high-threshold region, we further demonstrate that typical code realizations outperform the thresholds and subthreshold logical error rates, at finite bias, of the best-known translationally invariant codes. We demonstrate the practical relevance of these random CDSC families by constructing a translation-invariant CDSC belonging to a high-performance random CDSC family. We also show that our translation-invariant CDSC outperforms well-known translation-invariant CDSCs such as the XZZX and XY codes.

}, issn = {2691-3399}, doi = {http://dx.doi.org/10.1103/PRXQuantum.5.010347}, url = {https://arxiv.org/abs/2201.07802}, author = {Dua, Arpit and Kubica, Aleksander and Jiang, Liang and Flammia, Steven T. and Michael Gullans} } @article {3489, title = {Efficient and practical Hamiltonian simulation from time-dependent product formulas}, year = {2024}, month = {3/13/2024}, abstract = {

In this work we propose an approach for implementing time-evolution of a quantum system using product formulas. The quantum algorithms we develop have provably better scaling (in terms of gate complexity and circuit depth) than a naive application of well-known Trotter formulas, for systems where the evolution is determined by a Hamiltonian with different energy scales (i.e., one part is \"large\" and another part is \"small\"). Our algorithms generate a decomposition of the evolution operator into a product of simple unitaries that are directly implementable on a quantum computer. Although the theoretical scaling is suboptimal compared with state-of-the-art algorithms (e.g., quantum signal processing), the performance of the algorithms we propose is highly competitive in practice. We illustrate this via extensive numerical simulations for several models. For instance, in the strong-field regime of the 1D transverse-field Ising model, our algorithms achieve an improvement of one order of magnitude in both the system size and evolution time that can be simulated with a fixed budget of 1000 arbitrary 2-qubit gates, compared with standard Trotter formulas.

}, url = {https://arxiv.org/abs/2403.08729}, author = {Jan Lukas Bosse and Andrew M. Childs and Charles Derby and Filippo Maria Gambetta and Ashley Montanaro and Raul A. Santos} } @article {3490, title = {Efficiently verifiable quantum advantage on near-term analog quantum simulators}, year = {2024}, month = {3/13/2024}, abstract = {

Existing schemes for demonstrating quantum computational advantage are subject to various practical restrictions, including the hardness of verification and challenges in experimental implementation. Meanwhile, analog quantum simulators have been realized in many experiments to study novel physics. In this work, we propose a quantum advantage protocol based on single-step Feynman-Kitaev verification of an analog quantum simulation, in which the verifier need only run an O(λ2)-time classical computation, and the prover need only prepare O(1) samples of a history state and perform O(λ2) single-qubit measurements, for a security parameter λ. We also propose a near-term feasible strategy for honest provers and discuss potential experimental realizations.

}, url = {https://arxiv.org/abs/2403.08195}, author = {Zhenning Liu and Dhruv Devulapalli and Dominik Hangleiter and Yi-Kai Liu and Alicia J. Koll{\'a}r and Alexey V. Gorshkov and Andrew M. Childs} } @article {3462, title = {Estimation of Hamiltonian parameters from thermal states}, year = {2024}, month = {1/18/2024}, abstract = {

We upper- and lower-bound the optimal precision with which one can estimate an unknown Hamiltonian parameter via measurements of Gibbs thermal states with a known temperature. The bounds depend on the uncertainty in the Hamiltonian term that contains the parameter and on the term\&$\#$39;s degree of noncommutativity with the full Hamiltonian: higher uncertainty and commuting operators lead to better precision. We apply the bounds to show that there exist entangled thermal states such that the parameter can be estimated with an error that decreases faster than 1/n\−\−\√, beating the standard quantum limit. This result governs Hamiltonians where an unknown scalar parameter (e.g. a component of a magnetic field) is coupled locally and identically to n qubit sensors. In the high-temperature regime, our bounds allow for pinpointing the optimal estimation error, up to a constant prefactor. Our bounds generalize to joint estimations of multiple parameters. In this setting, we recover the high-temperature sample scaling derived previously via techniques based on quantum state discrimination and coding theory. In an application, we show that noncommuting conserved quantities hinder the estimation of chemical potentials.

}, url = {https://arxiv.org/abs/2401.10343}, author = {Luis Pedro Garc{\'\i}a-Pintos and Kishor Bharti and Jacob Bringewatt and Hossein Dehghani and Adam Ehrenberg and Nicole Yunger Halpern and Alexey V. Gorshkov} } @article {3503, title = {High-Energy Collision of Quarks and Mesons in the Schwinger Model: From Tensor Networks to Circuit QED}, journal = {Physical Review Letters}, volume = {132}, year = {2024}, month = {2/28/2024}, abstract = {

With the aim of studying nonperturbative out-of-equilibrium dynamics of high-energy particle collisions on quantum simulators, we investigate the scattering dynamics of lattice quantum electrodynamics in 1+1 dimensions. Working in the bosonized formulation of the model and in the thermodynamic limit, we use uniform-matrix-product-state tensor networks to construct multi-particle wave-packet states, evolve them in time, and detect outgoing particles post collision. This facilitates the numerical simulation of scattering experiments in both confined and deconfined regimes of the model at different energies, giving rise to rich phenomenology, including inelastic production of quark and meson states, meson disintegration, and dynamical string formation and breaking. We obtain elastic and inelastic scattering cross sections, together with time-resolved momentum and position distributions of the outgoing particles. Furthermore, we propose an analog circuit-QED implementation of the scattering process that is native to the platform, requires minimal ingredients and approximations, and enables practical schemes for particle wave-packet preparation and evolution. This study highlights the role of classical and quantum simulation in enhancing our understanding of scattering processes in quantum field theories in real time.

}, issn = {1079-7114}, doi = {http://dx.doi.org/10.1103/PhysRevLett.132.091903}, url = {https://arxiv.org/abs/2307.02522}, author = {Belyansky, Ron and Whitsitt, Seth and Mueller, Niklas and Fahimniya, Ali and Bennewitz, Elizabeth R. and Davoudi, Zohreh and Gorshkov, Alexey V.} } @article {3497, title = {Long-Distance Nuclear Matrix Elements for Neutrinoless Double-Beta Decay from Lattice QCD}, year = {2024}, month = {2/14/2024}, abstract = {

Neutrinoless double-beta (0νββ) decay is a heretofore unobserved process which, if observed, would imply that neutrinos are Majorana particles. Interpretations of the stringent experimental constraints on 0νββ-decay half-lives require calculations of nuclear matrix elements. This work presents the first lattice quantum-chromodynamics (LQCD) calculation of the matrix element for 0νββ decay in a multi-nucleon system, specifically the nn\→ppee transition, mediated by a light left-handed Majorana neutrino propagating over nuclear-scale distances. This calculation is performed with quark masses corresponding to a pion mass of mπ=806 MeV at a single lattice spacing and volume. The statistically cleaner Σ\−\→Σ+ee transition is also computed in order to investigate various systematic uncertainties. The prospects for matching the results of LQCD calculations onto a nuclear effective field theory to determine a leading-order low-energy constant relevant for 0νββ decay with a light Majorana neutrino are investigated. This work, therefore, sets the stage for future calculations at physical values of the quark masses that, combined with effective field theory and nuclear many-body studies, will provide controlled theoretical inputs to experimental searches of 0νββ decay.

}, url = {https://arxiv.org/abs/2402.09362}, author = {Zohreh Davoudi and William Detmold and Zhenghao Fu and Anthony V. Grebe and William Jay and David Murphy and Patrick Oare and Phiala E. Shanahan and Michael L. Wagman} } @article {3485, title = {Properties and Applications of the Kirkwood-Dirac Distribution}, year = {2024}, month = {3/27/2024}, abstract = {

The most famous quasi-probability distribution, the Wigner function, has played a pivotal role in the development of a continuous-variable quantum theory that has clear analogues of position and momentum. However, the Wigner function is ill-suited for much modern quantum-information research, which is focused on finite-dimensional systems and general observables. Instead, recent years have seen the Kirkwood-Dirac (KD) distribution come to the forefront as a powerful quasi-probability distribution for analysing quantum mechanics. The KD distribution allows tools from statistics and probability theory to be applied to problems in quantum-information processing. A notable difference to the Wigner function is that the KD distribution can represent a quantum state in terms of arbitrary observables. This paper reviews the KD distribution, in three parts. First, we present definitions and basic properties of the KD distribution and its generalisations. Second, we summarise the KD distribution\&$\#$39;s extensive usage in the study or development of measurement disturbance; quantum metrology; weak values; direct measurements of quantum states; quantum thermodynamics; quantum scrambling and out-of-time-ordered correlators; and the foundations of quantum mechanics, including Leggett-Garg inequalities, the consistent-histories interpretation, and contextuality. We emphasise connections between operational quantum advantages and negative or non-real KD quasi-probabilities. Third, we delve into the KD distribution\&$\#$39;s mathematical structure. We summarise the current knowledge regarding the geometry of KD-positive states (the states for which the KD distribution is a classical probability distribution), describe how to witness and quantify KD non-positivity, and outline relationships between KD non-positivity and observables\&$\#$39; incompatibility.

}, url = {https://arxiv.org/abs/2403.18899}, author = {David R. M. Arvidsson-Shukur and William F. Braasch Jr. and Stephan De Bievre and Justin Dressel and Andrew N. Jordan and Christopher Langrenez and Matteo Lostaglio and Jeff S. Lundeen and Nicole Yunger Halpern} } @article {3484, title = {Quantum thermodynamics of nonequilibrium processes in lattice gauge theories}, year = {2024}, month = {4/3/2024}, abstract = {

A key objective in nuclear and high-energy physics is to describe nonequilibrium dynamics of matter, e.g., in the early universe and in particle colliders, starting from the Standard Model. Classical-computing methods, via the framework of lattice gauge theory, have experienced limited success in this mission. Quantum simulation of lattice gauge theories holds promise for overcoming computational limitations. Because of local constraints (Gauss\&$\#$39;s laws), lattice gauge theories have an intricate Hilbert-space structure. This structure complicates the definition of thermodynamic properties of systems coupled to reservoirs during equilibrium and nonequilibrium processes. We show how to define thermodynamic quantities such as work and heat using strong-coupling thermodynamics, a framework that has recently burgeoned within the field of quantum thermodynamics. Our definitions suit instantaneous quenches, simple nonequilibrium processes undertaken in quantum simulators. To illustrate our framework, we compute the work and heat exchanged during a quench in a Z2 lattice gauge theory coupled to matter in 1+1 dimensions. The thermodynamic quantities, as functions of the quench parameter, evidence an expected phase transition. For general thermal states, we derive a simple relation between a quantum many-body system\&$\#$39;s entanglement Hamiltonian, measurable with quantum-information-processing tools, and the Hamiltonian of mean force, used to define strong-coupling thermodynamic quantities.

}, url = {https://arxiv.org/abs/2404.02965}, author = {Zohreh Davoudi and Christopher Jarzynski and Niklas Mueller and Greeshma Oruganti and Connor Powers and Nicole Yunger Halpern} } @article {3500, title = {Scattering wave packets of hadrons in gauge theories: Preparation on a quantum computer}, year = {2024}, month = {2/1/2024}, abstract = {

Quantum simulation holds promise of enabling a complete description of high-energy scattering processes rooted in gauge theories of the Standard Model. A first step in such simulations is preparation of interacting hadronic wave packets. To create the wave packets, one typically resorts to adiabatic evolution to bridge between wave packets in the free theory and those in the interacting theory, rendering the simulation resource intensive. In this work, we construct a wave-packet creation operator directly in the interacting theory to circumvent adiabatic evolution, taking advantage of resource-efficient schemes for ground-state preparation, such as variational quantum eigensolvers. By means of an ansatz for bound mesonic excitations in confining gauge theories, which is subsequently optimized using classical or quantum methods, we show that interacting mesonic wave packets can be created efficiently and accurately using digital quantum algorithms that we develop. Specifically, we obtain high-fidelity mesonic wave packets in the Z2 and U(1) lattice gauge theories coupled to fermionic matter in 1+1 dimensions. Our method is applicable to both perturbative and non-perturbative regimes of couplings. The wave-packet creation circuit for the case of the Z2 lattice gauge theory is built and implemented on the Quantinuum H1-1 trapped-ion quantum computer using 13 qubits and up to 308 entangling gates. The fidelities agree well with classical benchmark calculations after employing a simple symmetry-based noise-mitigation technique. This work serves as a step toward quantum computing scattering processes in quantum chromodynamics.

}, url = {https://arxiv.org/abs/2402.00840}, author = {Zohreh Davoudi and Chung-Chun Hsieh and Saurabh V. Kadam} } @article {3488, title = {The Second Moment of Hafnians in Gaussian Boson Sampling}, year = {2024}, month = {3/20/2024}, abstract = {

Gaussian Boson Sampling is a popular method for experimental demonstrations of quantum advantage, but many subtleties remain in fully understanding its theoretical underpinnings. An important component in the theoretical arguments for approximate average-case hardness of sampling is anticoncentration, which is a second-moment property of the output probabilities. In Gaussian Boson Sampling these are given by hafnians of generalized circular orthogonal ensemble matrices. In a companion work [arXiv:2312.08433], we develop a graph-theoretic method to study these moments and use it to identify a transition in anticoncentration. In this work, we find a recursive expression for the second moment using these graph-theoretic techniques. While we have not been able to solve this recursion by hand, we are able to solve it numerically exactly, which we do up to Fock sector 2n=80. We further derive new analytical results about the second moment. These results allow us to pinpoint the transition in anticoncentration and furthermore yield the expected linear cross-entropy benchmarking score for an ideal (error-free) device.

}, url = {https://arxiv.org/abs/2403.13878}, author = {Adam Ehrenberg and Joseph T. Iosue and Abhinav Deshpande and Dominik Hangleiter and Alexey V. Gorshkov} } @article {3491, title = {Simulating Meson Scattering on Spin Quantum Simulators}, year = {2024}, month = {3/11/2024}, abstract = {

Studying high-energy collisions of composite particles, such as hadrons and nuclei, is an outstanding goal for quantum simulators. However, preparation of hadronic wave packets has posed a significant challenge, due to the complexity of hadrons and the precise structure of wave packets. This has limited demonstrations of hadron scattering on quantum simulators to date. Observations of confinement and composite excitations in quantum spin systems have opened up the possibility to explore scattering dynamics in spin models. In this article, we develop two methods to create entangled spin states corresponding to wave packets of composite particles in analog quantum simulators of Ising spin Hamiltonians. One wave-packet preparation method uses the blockade effect enabled by beyond-nearest-neighbor Ising spin interactions. The other method utilizes a quantum-bus-mediated exchange, such as the native spin-phonon coupling in trapped-ion arrays. With a focus on trapped-ion simulators, we numerically benchmark both methods and show that high-fidelity wave packets can be achieved in near-term experiments. We numerically study scattering of wave packets for experimentally realizable parameters in the Ising model and find inelastic-scattering regimes, corresponding to particle production in the scattering event, with prominent and distinct experimental signals. Our proposal, therefore, demonstrates the potential of observing inelastic scattering in near-term quantum simulators.

}, url = {https://arxiv.org/abs/2403.07061}, author = {Elizabeth R. Bennewitz and Brayden Ware and Alexander Schuckert and Alessio Lerose and Federica M. Surace and Ron Belyansky and William Morong and De Luo and Arinjoy De and Kate S. Collins and Or Katz and Christopher Monroe and Zohreh Davoudi and Alexey V. Gorshkov} } @article {3257, title = {Accelerating Progress Towards Practical Quantum Advantage: The Quantum Technology Demonstration Project Roadmap}, year = {2023}, month = {3/20/2023}, abstract = {

Quantum information science and technology (QIST) is a critical and emerging technology with the potential for enormous world impact and is currently invested in by over 40 nations. To bring these large-scale investments to fruition and bridge the lower technology readiness levels (TRLs) of fundamental research at universities to the high TRLs necessary to realize the promise of practical quantum advantage accessible to industry and the public, we present a roadmap for Quantum Technology Demonstration Projects (QTDPs). Such QTDPs, focused on intermediate TRLs, are large-scale public-private partnerships with a high probability of translation from laboratory to practice. They create technology demonstrating a clear \&$\#$39;quantum advantage\&$\#$39; for science breakthroughs that are user-motivated and will provide access to a broad and diverse community of scientific users. Successful implementation of a program of QTDPs will have large positive economic impacts.

}, url = {https://arxiv.org/abs/2210.14757}, author = {Paul Alsing and Phil Battle and Joshua C. Bienfang and Tammie Borders and Tina Brower-Thomas and Lincoln D. Carr and Fred Chong and Siamak Dadras and Brian DeMarco and Ivan Deutsch and Eden Figueroa and Danna Freedman and Henry Everitt and Daniel Gauthier and Ezekiel Johnston-Halperin and Jungsang Kim and Mackillo Kira and Prem Kumar and Paul Kwiat and John Lekki and Anjul Loiacono and Marko Lon{\v c}ar and John R. Lowell and Mikhail Lukin and Celia Merzbacher and Aaron Miller and Christopher Monroe and Johannes Pollanen and David Pappas and Michael Raymer and Ronald Reano and Brandon Rodenburg and Martin Savage and Thomas Searles and Jun Ye} } @article {3421, title = {Digital quantum simulation of NMR experiments}, journal = {Science Advances}, volume = {9}, year = {2023}, month = {11/29/2023}, abstract = {

Simulations of nuclear magnetic resonance (NMR) experiments can be an important tool for extracting information about molecular structure and optimizing experimental protocols but are often intractable on classical computers for large molecules such as proteins and for protocols such as zero-field NMR. We demonstrate the first quantum simulation of an NMR spectrum, computing the zero-field spectrum of the methyl group of acetonitrile using four qubits of a trapped-ion quantum computer. We reduce the sampling cost of the quantum simulation by an order of magnitude using compressed sensing techniques. We show how the intrinsic decoherence of NMR systems may enable the zero-field simulation of classically hard molecules on relatively near-term quantum hardware and discuss how the experimentally demonstrated quantum algorithm can be used to efficiently simulate scientifically and technologically relevant solid-state NMR experiments on more mature devices. Our work opens a practical application for quantum computation.

}, issn = {2375-2548}, doi = {10.1126/sciadv.adh2594}, url = {https://arxiv.org/abs/2109.13298}, author = {Seetharam, Kushal and Biswas, Debopriyo and Noel, Crystal and Risinger, Andrew and Zhu, Daiwei and Katz, Or and Chattopadhyay, Sambuddha and Cetina, Marko and Monroe, Christopher and Demler, Eugene and Sels, Dries} } @article {3359, title = {Fault-tolerant hyperbolic Floquet quantum error correcting codes}, year = {2023}, month = {9/18/2023}, abstract = {

A central goal in quantum error correction is to reduce the overhead of fault-tolerant quantum computing by increasing noise thresholds and reducing the number of physical qubits required to sustain a logical qubit. We introduce a potential path towards this goal based on a family of dynamically generated quantum error correcting codes that we call \"hyperbolic Floquet codes.\" These codes are defined by a specific sequence of non-commuting two-body measurements arranged periodically in time that stabilize a topological code on a hyperbolic manifold with negative curvature. We focus on a family of lattices for n qubits that, according to our prescription that defines the code, provably achieve a finite encoding rate (1/8+2/n) and have a depth-3 syndrome extraction circuit. Similar to hyperbolic surface codes, the distance of the code at each time-step scales at most logarithmically in n. The family of lattices we choose indicates that this scaling is achievable in practice. We develop and benchmark an efficient matching-based decoder that provides evidence of a threshold near 0.1\% in a phenomenological noise model. Utilizing weight-two check operators and a qubit connectivity of 3, one of our hyperbolic Floquet codes uses 400 physical qubits to encode 52 logical qubits with a code distance of 8, i.e., it is a [[400,52,8]] code. At small error rates, comparable logical error suppression to this code requires 5x as many physical qubits (1924) when using the honeycomb Floquet code with the same noise model and decoder.

}, url = {https://arxiv.org/abs/2309.10033}, author = {Ali Fahimniya and Hossein Dehghani and Kishor Bharti and Sheryl Mathew and Alicia J. Koll{\'a}r and Alexey V. Gorshkov and Michael Gullans} } @conference {3370, title = {Fixing and Mechanizing the Security Proof of Fiat-Shamir with Aborts and Dilithium}, booktitle = {Advances in Cryptology {\textendash} CRYPTO 2023}, year = {2023}, month = {8/9/2023}, publisher = {Springer Nature Switzerland}, organization = {Springer Nature Switzerland}, address = {Cham}, abstract = {

We extend and consolidate the security justification for the Dilithium signature scheme. In particular, we identify a subtle but crucial gap that appears in several ROM and QROM security proofs for signature schemes that are based on the Fiat-Shamir with aborts paradigm, including Dilithium. The gap lies in the CMA-to-NMA reduction and was uncovered when trying to formalize a variant of the QROM security proof by Kiltz, Lyubashevsky, and Schaffner (Eurocrypt 2018). The gap was confirmed by the authors, and there seems to be no simple patch for it. We provide new, fixed proofs for the affected CMA-to-NMA reduction, both for the ROM and the QROM, and we perform a concrete security analysis for the case of Dilithium to show that the claimed security level is still valid after addressing the gap. Furthermore, we offer a fully mechanized ROM proof for the CMA-security of Dilithium in the EasyCrypt proof assistant. Our formalization includes several new tools and techniques of independent interest for future formal verification results.

}, isbn = {978-3-031-38554-4}, author = {Barbosa, Manuel and Barthe, Gilles and Doczkal, Christian and Don, Jelle and Fehr, Serge and Gr{\'e}goire, Benjamin and Huang, Yu-Hsuan and H{\"u}lsing, Andreas and Lee, Yi and Wu, Xiaodi}, editor = {Handschuh, Helena and Lysyanskaya, Anna} } @article {3419, title = {General quantum algorithms for Hamiltonian simulation with applications to a non-Abelian lattice gauge theory}, journal = {Quantum}, volume = {7}, year = {2023}, month = {12/14/2023}, pages = {1213}, abstract = {

With a focus on universal quantum computing for quantum simulation, and through the example of lattice gauge theories, we introduce rather general quantum algorithms that can efficiently simulate certain classes of interactions consisting of correlated changes in multiple (bosonic and fermionic) quantum numbers with non-trivial functional coefficients. In particular, we analyze diagonalization of Hamiltonian terms using a singular-value decomposition technique, and discuss how the achieved diagonal unitaries in the digitized time-evolution operator can be implemented. The lattice gauge theory studied is the SU(2) gauge theory in 1+1 dimensions coupled to one flavor of staggered fermions, for which a complete quantum-resource analysis within different computational models is presented. The algorithms are shown to be applicable to higher-dimensional theories as well as to other Abelian and non-Abelian gauge theories. The example chosen further demonstrates the importance of adopting efficient theoretical formulations: it is shown that an explicitly gauge-invariant formulation using loop, string, and hadron degrees of freedom simplifies the algorithms and lowers the cost compared with the standard formulations based on angular-momentum as well as the Schwinger-boson degrees of freedom. The loop-string-hadron formulation further retains the non-Abelian gauge symmetry despite the inexactness of the digitized simulation, without the need for costly controlled operations. Such theoretical and algorithmic considerations are likely to be essential in quantumly simulating other complex theories of relevance to nature.

}, issn = {2521-327X}, doi = {10.22331/q-2023-12-20-1213}, url = {https://arxiv.org/abs/2212.14030}, author = {Zohreh Davoudi and Shaw, Alexander F. and Stryker, Jesse R.} } @article {3305, title = {High-Energy Collision of Quarks and Hadrons in the Schwinger Model: From Tensor Networks to Circuit QED}, year = {2023}, month = {7/5/2023}, abstract = {

With the aim of studying nonperturbative out-of-equilibrium dynamics of high-energy particle collisions on quantum simulators, we investigate the scattering dynamics of lattice quantum electrodynamics in 1+1 dimensions. Working in the bosonized formulation of the model, we propose an analog circuit-QED implementation that is native to the platform, requires minimal ingredients and approximations, and enables practical schemes for particle wave-packet preparation and evolution. Furthermore, working in the thermodynamic limit, we use uniform-matrix-product-state tensor networks to construct multi-particle wave-packet states, evolve them in time, and detect outgoing particles post collision. This facilitates the numerical simulation of scattering experiments in both confined and deconfined regimes of the model at different energies, giving rise to rich phenomenology, including inelastic production of quark and meson states, meson disintegration, and dynamical string formation and breaking. We obtain elastic and inelastic scattering cross sections, together with time-resolved momentum and position distributions of the outgoing particles. This study highlights the role of classical and quantum simulation in enhancing our understanding of scattering processes in quantum field theories in real time.

}, url = {https://arxiv.org/abs/2307.02522}, author = {Ron Belyansky and Seth Whitsitt and Niklas Mueller and Ali Fahimniya and Elizabeth R. Bennewitz and Zohreh Davoudi and Alexey V. Gorshkov} } @article {3422, title = {Lattice quantum chromodynamics at large isospin density: 6144 pions in a box}, year = {2023}, month = {7/27/2023}, abstract = {

We present an algorithm to compute correlation functions for systems with the quantum numbers of many identical mesons from lattice quantum chromodynamics (QCD). The algorithm is numerically stable and allows for the computation of n-pion correlation functions for n\∈{1,\…,N} using a single N\×N matrix decomposition, improving on previous algorithms. We apply the algorithm to calculations of correlation functions with up to 6144 π+s using two ensembles of gauge field configurations generated with quark masses corresponding to a pion mass mπ=170 MeV and spacetime volumes of (4.43\×8.8) fm4 and (5.83\×11.6) fm4. We also discuss statistical techniques for the analysis of such systems, in which the correlation functions vary over many orders of magnitude. In particular, we observe that the many-pion correlation functions are well approximated by log-normal distributions, allowing the extraction of the energies of these systems. Using these energies, the large-isospin-density, zero-baryon-density region of the QCD phase diagram is explored. A peak is observed in the energy density at an isospin chemical potential μI\∼1.5mπ, signalling the transition into a Bose-Einstein condensed phase. The isentropic speed of sound in the medium is seen to exceed the ideal-gas (conformal) limit (c2s\≤1/3) over a wide range of chemical potential before falling towards the asymptotic expectation at μI\∼15mπ. These, and other thermodynamic observables, indicate that the isospin chemical potential must be large for the system to be well described by an ideal gas or perturbative QCD.

}, url = {https://arxiv.org/abs/2307.15014}, author = {Ryan Abbott and William Detmold and Fernando Romero-L{\'o}pez and Zohreh Davoudi and Marc Illa and Assumpta Parre{\~n}o and Robert J. Perry and Phiala E. Shanahan and Michael L. Wagman} } @article {3358, title = {Non-equilibrium critical scaling and universality in a quantum simulator}, year = {2023}, month = {9/19/2023}, abstract = {

Universality and scaling laws are hallmarks of equilibrium phase transitions and critical phenomena. However, extending these concepts to non-equilibrium systems is an outstanding challenge. Despite recent progress in the study of dynamical phases, the universality classes and scaling laws for non-equilibrium phenomena are far less understood than those in equilibrium. In this work, using a trapped-ion quantum simulator with single-ion resolution, we investigate the non-equilibrium nature of critical fluctuations following a quantum quench to the critical point. We probe the scaling of spin fluctuations after a series of quenches to the critical Hamiltonian of a long-range Ising model. With systems of up to 50 spins, we show that the amplitude and timescale of the post-quench fluctuations scale with system size with distinct universal critical exponents. While a generic quench can lead to thermal critical behaviour, we find that a second quench from one critical state to another (i.e. a double quench) results in critical behaviour that does not have an equilibrium counterpart. Our results demonstrate the ability of quantum simulators to explore universal scaling beyond the equilibrium paradigm.

}, url = {https://arxiv.org/abs/2309.10856}, author = {A. De and P. Cook and K. Collins and W. Morong and D. Paz and P. Titum and G. Pagano and Alexey V. Gorshkov and M. Maghrebi and C. Monroe} } @article {3413, title = {Observation of a finite-energy phase transition in a one-dimensional quantum simulator}, year = {2023}, month = {10/30/2023}, abstract = {

One of the most striking many-body phenomena in nature is the sudden change of macroscopic properties as the temperature or energy reaches a critical value. Such equilibrium transitions have been predicted and observed in two and three spatial dimensions, but have long been thought not to exist in one-dimensional (1D) systems. Fifty years ago, Dyson and Thouless pointed out that a phase transition in 1D can occur in the presence of long-range interactions, but an experimental realization has so far not been achieved due to the requirement to both prepare equilibrium states and realize sufficiently long-range interactions. Here we report on the first experimental demonstration of a finite-energy phase transition in 1D. We use the simple observation that finite-energy states can be prepared by time-evolving product initial states and letting them thermalize under the dynamics of a many-body Hamiltonian. By preparing initial states with different energies in a 1D trapped-ion quantum simulator, we study the finite-energy phase diagram of a long-range interacting quantum system. We observe a ferromagnetic equilibrium phase transition as well as a crossover from a low-energy polarized paramagnet to a high-energy unpolarized paramagnet in a system of up to 23 spins, in excellent agreement with numerical simulations. Our work demonstrates the ability of quantum simulators to realize and study previously inaccessible phases at finite energy density.

}, url = {https://arxiv.org/abs/2310.19869}, author = {Alexander Schuckert and Or Katz and Lei Feng and Eleanor Crane and Arinjoy De and Mohammad Hafezi and Alexey V. Gorshkov and Christopher Monroe} } @article {3315, title = {Page curves and typical entanglement in linear optics}, journal = {Quantum}, volume = {7}, year = {2023}, month = {5/18/2023}, pages = {1017}, abstract = {

Bosonic Gaussian states are a special class of quantum states in an infinite dimensional Hilbert space that are relevant to universal continuous-variable quantum computation as well as to near-term quantum sampling tasks such as Gaussian Boson Sampling. In this work, we study entanglement within a set of squeezed modes that have been evolved by a random linear optical unitary. We first derive formulas that are asymptotically exact in the number of modes for the R{\'e}nyi-2 Page curve (the average R{\'e}nyi-2 entropy of a subsystem of a pure bosonic Gaussian state) and the corresponding Page correction (the average information of the subsystem) in certain squeezing regimes. We then prove various results on the typicality of entanglement as measured by the R{\'e}nyi-2 entropy by studying its variance. Using the aforementioned results for the R{\'e}nyi-2 entropy, we upper and lower bound the von Neumann entropy Page curve and prove certain regimes of entanglement typicality as measured by the von Neumann entropy. Our main proofs make use of a symmetry property obeyed by the average and the variance of the entropy that dramatically simplifies the averaging over unitaries. In this light, we propose future research directions where this symmetry might also be exploited. We conclude by discussing potential applications of our results and their generalizations to Gaussian Boson Sampling and to illuminating the relationship between entanglement and computational complexity.

}, doi = {10.22331/q-2023-05-23-1017}, url = {https://arxiv.org/abs/2209.06838}, author = {Joseph T. Iosue and Adam Ehrenberg and Dominik Hangleiter and Abhinav Deshpande and Alexey V. Gorshkov} } @article {3262, title = {Parallelization techniques for quantum simulation of fermionic systems}, year = {2023}, month = {3/30/2023}, abstract = {

Mapping fermionic operators to qubit operators is an essential step for simulating fermionic systems on a quantum computer. We investigate how the choice of such a mapping interacts with the underlying qubit connectivity of the quantum processor to enable (or impede) parallelization of the resulting Hamiltonian-simulation algorithm. It is shown that this problem can be mapped to a path coloring problem on a graph constructed from the particular choice of encoding fermions onto qubits and the fermionic interactions onto paths. The basic version of this problem is called the weak coloring problem. Taking into account the fine-grained details of the mapping yields what is called the strong coloring problem, which leads to improved parallelization performance. A variety of illustrative analytical and numerical examples are presented to demonstrate the amount of improvement for both weak and strong coloring-based parallelizations. Our results are particularly important for implementation on near-term quantum processors where minimizing circuit depth is necessary for algorithmic feasibility.

}, url = {https://arxiv.org/abs/2207.12470}, author = {Jacob Bringewatt and Zohreh Davoudi} } @article {3400, title = {Quantum Algorithms for Simulating Nuclear Effective Field Theories}, year = {2023}, month = {12/8/2023}, abstract = {

Quantum computers offer the potential to simulate nuclear processes that are classically intractable. With the goal of understanding the necessary quantum resources, we employ state-of-the-art Hamiltonian-simulation methods, and conduct a thorough algorithmic analysis, to estimate the qubit and gate costs to simulate low-energy effective field theories (EFTs) of nuclear physics. In particular, within the framework of nuclear lattice EFT, we obtain simulation costs for the leading-order pionless and pionful EFTs. We consider both static pions represented by a one-pion-exchange potential between the nucleons, and dynamical pions represented by relativistic bosonic fields coupled to non-relativistic nucleons. We examine the resource costs for the tasks of time evolution and energy estimation for physically relevant scales. We account for model errors associated with truncating either long-range interactions in the one-pion-exchange EFT or the pionic Hilbert space in the dynamical-pion EFT, and for algorithmic errors associated with product-formula approximations and quantum phase estimation. Our results show that the pionless EFT is the least costly to simulate and the dynamical-pion theory is the costliest. We demonstrate how symmetries of the low-energy nuclear Hamiltonians can be utilized to obtain tighter error bounds on the simulation algorithm. By retaining the locality of nucleonic interactions when mapped to qubits, we achieve reduced circuit depth and substantial parallelization. We further develop new methods to bound the algorithmic error for classes of fermionic Hamiltonians that preserve the number of fermions, and demonstrate that reasonably tight Trotter error bounds can be achieved by explicitly computing nested commutators of Hamiltonian terms. This work highlights the importance of combining physics insights and algorithmic advancement in reducing quantum-simulation costs.

}, url = {https://arxiv.org/abs/2312.05344}, author = {James D. Watson and Jacob Bringewatt and Alexander F. Shaw and Andrew M. Childs and Alexey V. Gorshkov and Zohreh Davoudi} } @article {3368, title = {Quantum computation of dynamical quantum phase transitions and entanglement tomography in a lattice gauge theory}, year = {2023}, month = {10/6/2023}, abstract = {

Strongly-coupled gauge theories far from equilibrium may exhibit unique features that could illuminate the physics of the early universe and of hadron and ion colliders. Studying real-time phenomena has proven challenging with classical-simulation methods, but is a natural application of quantum simulation. To demonstrate this prospect, we quantum compute non-equal time correlation functions and perform entanglement tomography of non-equilibrium states of a simple lattice gauge theory, the Schwinger model, using a trapped-ion quantum computer by IonQ Inc. As an ideal target for near-term devices, a recently-predicted [Zache et al., Phys. Rev. Lett. 122, 050403 (2019)] dynamical quantum phase transition in this model is studied by preparing, quenching, and tracking the subsequent non-equilibrium dynamics in three ways: i) overlap echos signaling dynamical transitions, ii) non-equal time correlation functions with an underlying topological nature, and iii) the entanglement structure of non-equilibrium states, including entanglement Hamiltonians. These results constitute the first observation of a dynamical quantum phase transition in a lattice gauge theory on a quantum computer, and are a first step toward investigating topological phenomena in nuclear and high-energy physics using quantum technologies.

}, url = {https://arxiv.org/abs/2210.03089}, author = {Niklas Mueller and Joseph Carolan and Andrew Connelly and Zohreh Davoudi and Eugene F. Dumitrescu and K{\"u}bra Yeter-Aydeniz} } @article {3355, title = {Quantum Sensing with Erasure Qubits}, year = {2023}, month = {10/2/2023}, abstract = {

The dominant noise in an \"erasure qubit\" is an erasure -- a type of error whose occurrence and location can be detected. Erasure qubits have potential to reduce the overhead associated with fault tolerance. To date, research on erasure qubits has primarily focused on quantum computing and quantum networking applications. Here, we consider the applicability of erasure qubits to quantum sensing and metrology. We show theoretically that, for the same level of noise, an erasure qubit acts as a more precise sensor or clock compared to its non-erasure counterpart. We experimentally demonstrate this by artificially injecting either erasure errors (in the form of atom loss) or dephasing errors into a differential optical lattice clock comparison, and observe enhanced precision in the case of erasure errors for the same injected error rate. Similar benefits of erasure qubits to sensing can be realized in other quantum platforms like Rydberg atoms and superconducting qubits

}, url = {https://arxiv.org/abs/2310.01512}, author = {Pradeep Niroula and Jack Dolde and Xin Zheng and Jacob Bringewatt and Adam Ehrenberg and Kevin C. Cox and Jeff Thompson and Michael Gullans and Shimon Kolkowitz and Alexey V. Gorshkov} } @article {3479, title = {Quantum simulation of fundamental particles and forces}, journal = {Nature Reviews Physics}, volume = {5}, year = {2023}, month = {4/9/2024}, pages = {420{\textendash}432}, abstract = {

Simulating key static and dynamic properties of matter -- from creation in the Big Bang to evolution into sub-atomic and astrophysical environments -- arising from the underlying fundamental quantum fields of the Standard Model and their effective descriptions, lies beyond the capabilities of classical computation alone. Advances in quantum technologies have improved control over quantum entanglement and coherence to the point where robust simulations are anticipated to be possible in the foreseeable future. We discuss the emerging area of quantum simulations of Standard-Model physics, challenges that lie ahead, and opportunities for progress in the context of nuclear and high-energy physics.

}, issn = {2522-5820}, doi = {https://doi.org/10.1038/s42254-023-00599-8}, url = {https://arxiv.org/abs/2404.06298}, author = {Christian W. Bauer and Zohreh Davoudi and Natalie Klco and Martin J. Savage} } @article {3397, title = {Quantum-centric Supercomputing for Materials Science: A Perspective on Challenges and Future Directions}, year = {2023}, month = {12/14/2023}, abstract = {

Computational models are an essential tool for the design, characterization, and discovery of novel materials. Hard computational tasks in materials science stretch the limits of existing high-performance supercomputing centers, consuming much of their simulation, analysis, and data resources. Quantum computing, on the other hand, is an emerging technology with the potential to accelerate many of the computational tasks needed for materials science. In order to do that, the quantum technology must interact with conventional high-performance computing in several ways: approximate results validation, identification of hard problems, and synergies in quantum-centric supercomputing. In this paper, we provide a perspective on how quantum-centric supercomputing can help address critical computational problems in materials science, the challenges to face in order to solve representative use cases, and new suggested directions.

}, url = {https://arxiv.org/abs/2312.09733}, author = {Yuri Alexeev and Maximilian Amsler and Paul Baity and Marco Antonio Barroca and Sanzio Bassini and Torey Battelle and Daan Camps and David Casanova and Young jai Choi and Frederic T. Chong and Charles Chung and Chris Codella and Antonio D. Corcoles and James Cruise and Alberto Di Meglio and Jonathan Dubois and Ivan Duran and Thomas Eckl and Sophia Economou and Stephan Eidenbenz and Bruce Elmegreen and Clyde Fare and Ismael Faro and Cristina Sanz Fern{\'a}ndez and Rodrigo Neumann Barros Ferreira and Keisuke Fuji and Bryce Fuller and Laura Gagliardi and Giulia Galli and Jennifer R. Glick and Isacco Gobbi and Pranav Gokhale and Salvador de la Puente Gonzalez and Johannes Greiner and Bill Gropp and Michele Grossi and Emmanuel Gull and Burns Healy and Benchen Huang and Travis S. Humble and Nobuyasu Ito and Artur F. Izmaylov and Ali Javadi-Abhari and Douglas Jennewein and Shantenu Jha and Liang Jiang and Barbara Jones and Wibe Albert de Jong and Petar Jurcevic and William Kirby and Stefan Kister and Masahiro Kitagawa and Joel Klassen and Katherine Klymko and Kwangwon Koh and Masaaki Kondo and Doga Murat Kurkcuoglu and Krzysztof Kurowski and Teodoro Laino and Ryan Landfield and Matt Leininger and Vicente Leyton-Ortega and Ang Li and Meifeng Lin and Junyu Liu and Nicolas Lorente and Andre Luckow and Simon Martiel and Francisco Martin-Fernandez and Margaret Martonosi and Claire Marvinney and Arcesio Castaneda Medina and Dirk Merten and Antonio Mezzacapo and Kristel Michielsen and Abhishek Mitra and Tushar Mittal and Kyungsun Moon and Joel Moore and Mario Motta and Young-Hye Na and Yunseong Nam and Prineha Narang and Yu-ya Ohnishi and Daniele Ottaviani and Matthew Otten and Scott Pakin and Vincent R. Pascuzzi and Ed Penault and Tomasz Piontek and Jed Pitera and Patrick Rall and Gokul Subramanian Ravi and Niall Robertson and Matteo Rossi and Piotr Rydlichowski and Hoon Ryu and Georgy Samsonidze and Mitsuhisa Sato and Nishant Saurabh and Vidushi Sharma and Kunal Sharma and Soyoung Shin and George Slessman and Mathias Steiner and Iskandar Sitdikov and In-Saeng Suh and Eric Switzer and Wei Tang and Joel Thompson and Synge Todo and Minh Tran and Dimitar Trenev and Christian Trott and Huan-Hsin Tseng and Esin Tureci and David Garc{\'\i}a Valinas and Sofia Vallecorsa and Christopher Wever and Konrad Wojciechowski and Xiaodi Wu and Shinjae Yoo and Nobuyuki Yoshioka and Victor Wen-zhe Yu and Seiji Yunoki and Sergiy Zhuk and Dmitry Zubarev} } @article {3312, title = {A sharp phase transition in linear cross-entropy benchmarking}, year = {2023}, month = {5/8/2023}, abstract = {

Demonstrations of quantum computational advantage and benchmarks of quantum processors via quantum random circuit sampling are based on evaluating the linear cross-entropy benchmark (XEB). A key question in the theory of XEB is whether it approximates the fidelity of the quantum state preparation. Previous works have shown that the XEB generically approximates the fidelity in a regime where the noise rate per qudit ε satisfies εN<<1 for a system of N qudits and that this approximation breaks down at large noise rates. Here, we show that the breakdown of XEB as a fidelity proxy occurs as a sharp phase transition at a critical value of εN that depends on the circuit architecture and properties of the two-qubit gates, including in particular their entangling power. We study the phase transition using a mapping of average two-copy quantities to statistical mechanics models in random quantum circuit architectures with full or one-dimensional connectivity. We explain the phase transition behavior in terms of spectral properties of the transfer matrix of the statistical mechanics model and identify two-qubit gate sets that exhibit the largest noise robustness.

}, url = {https://arxiv.org/abs/2305.04954}, author = {Brayden Ware and Abhinav Deshpande and Dominik Hangleiter and Pradeep Niroula and Bill Fefferman and Alexey V. Gorshkov and Michael Gullans} } @article {3254, title = {Symphony: Expressive Secure Multiparty Computation with Coordination}, journal = {The Art, Science, and Engineering of Programming}, volume = {7}, year = {2023}, month = {2/20/2023}, type = {14}, abstract = {

Context: Secure Multiparty Computation (MPC) refers to a family of cryptographic techniques where mutually untrusting parties may compute functions of their private inputs while revealing only the function output. Inquiry: It can be hard to program MPCs correctly and efficiently using existing languages and frameworks, especially when they require coordinating disparate computational roles. How can we make this easier? Approach: We present Symphony, a new functional programming language for MPCs among two or more parties. Symphony starts from the single-instruction, multiple-data (SIMD) semantics of prior MPC languages, in which each party carries out symmetric responsibilities, and generalizes it using constructs that can coordinate many parties. Symphony introduces **first-class shares** and **first-class party sets** to provide unmatched language-level expressive power with high efficiency. Knowledge: Developing a core formal language called λ-Symphony, we prove that the intuitive, generalized SIMD view of a program coincides with its actual distributed semantics. Thus the programmer can reason about her programs by reading them from top to bottom, even though in reality the program runs in a coordinated fashion, distributed across many machines. We implemented a prototype interpreter for Symphony leveraging multiple cryptographic backends. With it we wrote a variety of MPC programs, finding that Symphony can express optimized protocols that other languages cannot, and that in general Symphony programs operate efficiently. [ full abstract at https://doi.org/10.22152/programming-journal.org/2023/7/14 ]\ 

}, doi = {10.22152/programming-journal.org/2023/7/14}, url = {https://arxiv.org/abs/2302.10076}, author = {Ian Sweet and David Darais and David Heath and William Harris and Ryan Estes and Michael Hicks} } @article {3399, title = {Transition of Anticoncentration in Gaussian Boson Sampling}, year = {2023}, month = {12/13/2023}, abstract = {

Gaussian Boson Sampling is a promising method for experimental demonstrations of quantum advantage because it is easier to implement than other comparable schemes. While most of the properties of Gaussian Boson Sampling are understood to the same degree as for these other schemes, we understand relatively little about the statistical properties of its output distribution. The most relevant statistical property, from the perspective of demonstrating quantum advantage, is the anticoncentration of the output distribution as measured by its second moment. The degree of anticoncentration features in arguments for the complexity-theoretic hardness of Gaussian Boson Sampling, and it is also important to know when using cross-entropy benchmarking to verify experimental performance. In this work, we develop a graph-theoretic framework for analyzing the moments of the Gaussian Boson Sampling distribution. Using this framework, we show that Gaussian Boson Sampling undergoes a transition in anticoncentration as a function of the number of modes that are initially squeezed compared to the number of photons measured at the end of the circuit. When the number of initially squeezed modes scales sufficiently slowly with the number of photons, there is a lack of anticoncentration. However, if the number of initially squeezed modes scales quickly enough, the output probabilities anticoncentrate weakly.

}, url = {https://arxiv.org/abs/2312.08433}, author = {Adam Ehrenberg and Joseph T. Iosue and Abhinav Deshpande and Dominik Hangleiter and Alexey V. Gorshkov} } @article {3001, title = {Approximating Output Probabilities of Shallow Quantum Circuits which are Geometrically-local in any Fixed Dimension}, journal = {Leibniz International Proceedings in Informatics (LIPIcs)}, volume = {232}, year = {2022}, month = {4/7/2022}, pages = {9:1--9:17}, type = {17th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2022)}, abstract = {

We present a classical algorithm that, for any D-dimensional geometrically-local, quantum circuit C of polylogarithmic-depth, and any bit string x\∈0,1n, can compute the quantity |\<x|C|0\⊗n\>|2 to within any inverse-polynomial additive error in quasi-polynomial time, for any fixed dimension D. This is an extension of the result [CC21], which originally proved this result for D=3. To see why this is interesting, note that, while the D=1 case of this result follows from standard use of Matrix Product States, known for decades, the D=2 case required novel and interesting techniques introduced in [BGM19]. Extending to the case D=3 was even more laborious and required further new techniques introduced in [CC21]. Our work here shows that, while handling each new dimension has historically required a new insight, and fixed algorithmic primitive, based on known techniques for D\≤3, we can now handle any fixed dimension D\>3.
Our algorithm uses the Divide-and-Conquer framework of [CC21] to approximate the desired quantity via several instantiations of the same problem type, each involving D-dimensional circuits on about half the number of qubits as the original. This division step is then applied recursively, until the width of the recursively decomposed circuits in the Dth dimension is so small that they can effectively be regarded as (D\−1)-dimensional problems by absorbing the small width in the Dth dimension into the qudit structure at the cost of a moderate increase in runtime. The main technical challenge lies in ensuring that the more involved portions of the recursive circuit decomposition and error analysis from [CC21] still hold in higher dimensions, which requires small modifications to the analysis in some places.

}, keywords = {Computational Complexity (cs.CC), FOS: Computer and information sciences, FOS: Physical sciences, Quantum Physics (quant-ph)}, isbn = {978-3-95977-237-2}, issn = {1868-8969}, doi = {10.48550/ARXIV.2202.08349}, url = {https://arxiv.org/abs/2202.08349}, author = {Dontha, Suchetan and Tan, Shi Jie Samuel and Smith, Stephen and Choi, Sangheon and Matthew Coudron} } @article {3128, title = {CoVault: A Secure Analytics Platform}, year = {2022}, month = {8/7/2022}, abstract = {

In a secure analytics platform, data sources consent to the exclusive use of their data for a pre-defined set of analytics queries performed by a specific group of analysts, and for a limited period. If the platform is secure under a sufficiently strong threat model, it can provide the missing link to enabling powerful analytics of sensitive personal data, by alleviating data subjects\&$\#$39; concerns about leakage and misuse of data. For instance, many types of powerful analytics that benefit public health, mobility, infrastructure, finance, or sustainable energy can be made differentially private, thus alleviating concerns about privacy. However, no platform currently exists that is sufficiently secure to alleviate concerns about data leakage and misuse; as a result, many types of analytics that would be in the interest of data subjects and the public are not done. CoVault uses a new multi-party implementation of functional encryption (FE) for secure analytics, which relies on a unique combination of secret sharing, multi-party secure computation (MPC), and different trusted execution environments (TEEs). CoVault is secure under a very strong threat model that tolerates compromise and side-channel attacks on any one of a small set of parties and their TEEs. Despite the cost of MPC, we show that CoVault scales to very large data sizes using map-reduce based query parallelization. For example, we show that CoVault can perform queries relevant to epidemic analytics at scale.

}, keywords = {and Cluster Computing (cs.DC), Cryptography and Security (cs.CR), Distributed, FOS: Computer and information sciences, Parallel}, doi = {10.48550/ARXIV.2208.03784}, url = {https://arxiv.org/abs/2208.03784}, author = {De Viti, Roberta and Sheff, Isaac and Glaeser, Noemi and Dinis, Baltasar and Rodrigues, Rodrigo and Katz, Jonathan and Bhattacharjee, Bobby and Hithnawi, Anwar and Garg, Deepak and Druschel, Peter} } @article {3205, title = {Efficient and practical quantum compiler towards multi-qubit systems with deep reinforcement learning}, year = {2022}, month = {4/14/2022}, abstract = {

Efficient quantum compiling tactics greatly enhance the capability of quantum computers to execute complicated quantum algorithms. Due to its fundamental importance, a plethora of quantum compilers has been designed in past years. However, there are several caveats to current protocols, which are low optimality, high inference time, limited scalability, and lack of universality. To compensate for these defects, here we devise an efficient and practical quantum compiler assisted by advanced deep reinforcement learning (RL) techniques, i.e., data generation, deep Q-learning, and AQ* search. In this way, our protocol is compatible with various quantum machines and can be used to compile multi-qubit operators. We systematically evaluate the performance of our proposal in compiling quantum operators with both inverse-closed and inverse-free universal basis sets. In the task of single-qubit operator compiling, our proposal outperforms other RL-based quantum compilers in the measure of compiling sequence length and inference time. Meanwhile, the output solution is near-optimal, guaranteed by the Solovay-Kitaev theorem. Notably, for the inverse-free universal basis set, the achieved sequence length complexity is comparable with the inverse-based setting and dramatically advances previous methods. These empirical results contribute to improving the inverse-free Solovay-Kitaev theorem. In addition, for the first time, we demonstrate how to leverage RL-based quantum compilers to accomplish two-qubit operator compiling. The achieved results open an avenue for integrating RL with quantum compiling to unify efficiency and practicality and thus facilitate the exploration of quantum advantages.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Machine Learning (cs.LG), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.06904}, url = {https://arxiv.org/abs/2204.06904}, author = {Chen, Qiuhao and Du, Yuxuan and Zhao, Qi and Jiao, Yuling and Lu, Xiliang and Wu, Xingyao} } @article {2607, title = {Implementing a Fast Unbounded Quantum Fanout Gate Using Power-Law Interactions}, journal = {Phys. Rev. Research}, volume = {4}, year = {2022}, month = {10/27/2022}, abstract = {

The standard circuit model for quantum computation presumes the ability to directly perform gates between arbitrary pairs of qubits, which is unlikely to be practical for large-scale experiments. Power-law interactions with strength decaying as 1/rα in the distance r provide an experimentally realizable resource for information processing, whilst still retaining long-range connectivity. We leverage the power of these interactions to implement a fast quantum fanout gate with an arbitrary number of targets. Our implementation allows the quantum Fourier transform (QFT) and Shor\&$\#$39;s algorithm to be performed on a D-dimensional lattice in time logarithmic in the number of qubits for interactions with α\≤D. As a corollary, we show that power-law systems with α\≤D are difficult to simulate classically even for short times, under a standard assumption that factoring is classically intractable. Complementarily, we develop a new technique to give a general lower bound, linear in the size of the system, on the time required to implement the QFT and the fanout gate in systems that are constrained by a linear light cone. This allows us to prove an asymptotically tighter lower bound for long-range systems than is possible with previously available techniques.\ 

}, doi = {https://doi.org/10.1103/PhysRevResearch.4.L042016}, url = {https://arxiv.org/abs/2007.00662}, author = {Andrew Y. Guo and Abhinav Deshpande and Su-Kuan Chu and Zachary Eldredge and Przemyslaw Bienias and Dhruv Devulapalli and Yuan Su and Andrew M. Childs and Alexey V. Gorshkov} } @article {3188, title = {Importance of the Spectral gap in Estimating Ground-State Energies}, journal = {PRX Quantum}, volume = {3}, year = {2022}, month = {12/9/2022}, abstract = {

The field of quantum Hamiltonian complexity lies at the intersection of quantum many-body physics and computational complexity theory, with deep implications to both fields. The main object of study is the LocalHamiltonian problem, which is concerned with estimating the ground-state energy of a local Hamiltonian and is complete for the class QMA, a quantum generalization of the class NP. A major challenge in the field is to understand the complexity of the LocalHamiltonian problem in more physically natural parameter regimes. One crucial parameter in understanding the ground space of any Hamiltonian in many-body physics is the spectral gap, which is the difference between the smallest two eigenvalues. Despite its importance in quantum many-body physics, the role played by the spectral gap in the complexity of the LocalHamiltonian is less well-understood. In this work, we make progress on this question by considering the precise regime, in which one estimates the ground-state energy to within inverse exponential precision. Computing ground-state energies precisely is a task that is important for quantum chemistry and quantum many-body physics.
In the setting of inverse-exponential precision, there is a surprising result that the complexity of LocalHamiltonian is magnified from QMA to PSPACE, the class of problems solvable in polynomial space. We clarify the reason behind this boost in complexity. Specifically, we show that the full complexity of the high precision case only comes about when the spectral gap is exponentially small. As a consequence of the proof techniques developed to show our results, we uncover important implications for the representability and circuit complexity of ground states of local Hamiltonians, the theory of uniqueness of quantum witnesses, and techniques for the amplification of quantum witnesses in the presence of postselection.

}, doi = {10.1103/prxquantum.3.040327}, url = {https://arxiv.org/abs/2007.11582}, author = {Abhinav Deshpande and Alexey V. Gorshkov and Bill Fefferman} } @article {3024, title = {Monitoring-induced Entanglement Entropy and Sampling Complexity}, year = {2022}, month = {1/29/2022}, abstract = {

The dynamics of open quantum systems is generally described by a master equation, which describes the loss of information into the environment. By using a simple model of uncoupled emitters, we illustrate how the recovery of this information depends on the monitoring scheme applied to register the decay clicks. The dissipative dynamics, in this case, is described by pure-state stochastic trajectories and we examine different unravelings of the same master equation. More precisely, we demonstrate how registering the sequence of clicks from spontaneously emitted photons through a linear optical interferometer induces entanglement in the trajectory states. Since this model consists of an array of single-photon emitters, we show a direct equivalence with Fock-state boson sampling and link the hardness of sampling the outcomes of the quantum jumps with the scaling of trajectory entanglement.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2201.12672}, url = {https://arxiv.org/abs/2201.12672}, author = {Van Regemortel, Mathias and Shtanko, Oles and Garc{\'\i}a-Pintos, Luis Pedro and Deshpande, Abhinav and Dehghani, Hossein and Alexey V. Gorshkov and Hafezi, Mohammad} } @article {3129, title = {Optical conductivity and orbital magnetization of Floquet vortex states}, year = {2022}, month = {4/20/2022}, abstract = {

Motivated by recent experimental demonstrations of Floquet topological insulators, there have been several theoretical proposals for using structured light, either spatial or spectral, to create other properties such as flat band and vortex states. In particular, the generation of vortex states in a massive Dirac fermion insulator irradiated by light carrying nonzero orbital angular momentum (OAM) has been proposed [Kim et al. Phys. Rev. B 105, L081301(2022)]. Here, we evaluate the orbital magnetization and optical conductivity as physical observables for such a system. We show that the OAM of light induces nonzero orbital magnetization and current density. The orbital magnetization density increases linearly as a function of OAM degree. In certain regimes, we find that orbital magnetization density is independent of the system size, width, and Rabi frequency of light. It is shown that the orbital magnetization arising from our Floquet theory is large and can be probed by magnetometry measurements. Furthermore, we study the optical conductivity for various types of electron transitions between different states such as vortex, edge, and bulk that are present in the system. Based on conductance frequency peaks, a scheme for the detection of vortex states is proposed.

}, keywords = {FOS: Physical sciences, Mesoscale and Nanoscale Physics (cond-mat.mes-hall), Other Condensed Matter (cond-mat.other)}, doi = {10.48550/ARXIV.2204.09488}, url = {https://arxiv.org/abs/2204.09488}, author = {Ahmadabadi, Iman and Dehghani, Hossein and Hafezi, Mohammad} } @article {3015, title = {Pauli Stabilizer Models of Twisted Quantum Doubles}, journal = {PRX Quantum}, volume = {3}, year = {2022}, month = {3/30/2022}, abstract = {

We construct a Pauli stabilizer model for every two-dimensional Abelian topological order that admits a gapped boundary. Our primary example is a Pauli stabilizer model on four-dimensional qudits that belongs to the double semion (DS) phase of matter. The DS stabilizer Hamiltonian is constructed by condensing an emergent boson in a Z4 toric code, where the condensation is implemented by making certain two-body measurements. We rigorously verify the topological order of the DS stabilizer model by identifying an explicit finite-depth quantum circuit (with ancillary qubits) that maps its ground state subspace to that of a DS string-net model. We show that the construction of the DS stabilizer Hamiltonian generalizes to all twisted quantum doubles (TQDs) with Abelian anyons. This yields a Pauli stabilizer code on composite-dimensional qudits for each such TQD, implying that the classification of topological Pauli stabilizer codes extends well beyond stacks of toric codes - in fact, exhausting all Abelian anyon theories that admit a gapped boundary. We also demonstrate that symmetry-protected topological phases of matter characterized by type I and type II cocycles can be modeled by Pauli stabilizer Hamiltonians by gauging certain 1-form symmetries of the TQD stabilizer models.

}, doi = {https://doi.org/10.1103\%2Fprxquantum.3.010353}, url = {https://arxiv.org/abs/2112.11394}, author = {Tyler D. Ellison and Yu-An Chen and Arpit Dua and Wilbur Shirley and Nathanan Tantivasadakarn and Dominic J. Williamson} } @article {3193, title = {Pauli topological subsystem codes from Abelian anyon theories}, year = {2022}, month = {11/7/2022}, abstract = {

We construct Pauli topological subsystem codes characterized by arbitrary two-dimensional Abelian anyon theories--this includes anyon theories with degenerate braiding relations and those without a gapped boundary to the vacuum. Our work both extends the classification of two-dimensional Pauli topological subsystem codes to systems of composite-dimensional qudits and establishes that the classification is at least as rich as that of Abelian anyon theories. We exemplify the construction with topological subsystem codes defined on four-dimensional qudits based on the Z(1)4 anyon theory with degenerate braiding relations and the chiral semion theory--both of which cannot be captured by topological stabilizer codes. The construction proceeds by \"gauging out\" certain anyon types of a topological stabilizer code. This amounts to defining a gauge group generated by the stabilizer group of the topological stabilizer code and a set of anyonic string operators for the anyon types that are gauged out. The resulting topological subsystem code is characterized by an anyon theory containing a proper subset of the anyons of the topological stabilizer code. We thereby show that every Abelian anyon theory is a subtheory of a stack of toric codes and a certain family of twisted quantum doubles that generalize the double semion anyon theory. We further prove a number of general statements about the logical operators of translation invariant topological subsystem codes and define their associated anyon theories in terms of higher-form symmetries.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2211.03798}, url = {https://arxiv.org/abs/2211.03798}, author = {Ellison, Tyler D. and Chen, Yu-An and Dua, Arpit and Shirley, Wilbur and Tantivasadakarn, Nathanan and Williamson, Dominic J.} } @article {2983, title = {Quantum computational advantage via high-dimensional Gaussian boson sampling}, journal = {Science Advances}, volume = {8}, year = {2022}, month = {1/5/2022}, pages = {eabi7894}, abstract = {

A programmable quantum computer based on fiber optics outperforms classical computers with a high level of confidence. Photonics is a promising platform for demonstrating a quantum computational advantage (QCA) by outperforming the most powerful classical supercomputers on a well-defined computational task. Despite this promise, existing proposals and demonstrations face challenges. Experimentally, current implementations of Gaussian boson sampling (GBS) lack programmability or have prohibitive loss rates. Theoretically, there is a comparative lack of rigorous evidence for the classical hardness of GBS. In this work, we make progress in improving both the theoretical evidence and experimental prospects. We provide evidence for the hardness of GBS, comparable to the strongest theoretical proposals for QCA. We also propose a QCA architecture we call high-dimensional GBS, which is programmable and can be implemented with low loss using few optical components. We show that particular algorithms for simulating GBS are outperformed by high-dimensional GBS experiments at modest system sizes. This work thus opens the path to demonstrating QCA with programmable photonic processors.

}, doi = {10.1126/sciadv.abi7894}, url = {https://www.science.org/doi/abs/10.1126/sciadv.abi7894}, author = {Abhinav Deshpande and Arthur Mehta and Trevor Vincent and Nicolas Quesada and Marcel Hinsche and Marios Ioannou and Lars Madsen and Jonathan Lavoie and Haoyu Qi and Jens Eisert and Dominik Hangleiter and Bill Fefferman and Ish Dhand} } @article {2996, title = {Quantum Routing with Teleportation}, year = {2022}, month = {4/8/2022}, abstract = {

We study the problem of implementing arbitrary permutations of qubits under interaction constraints in quantum systems that allow for arbitrarily fast local operations and classical communication (LOCC). In particular, we show examples of speedups over swap-based and more general unitary routing methods by distributing entanglement and using LOCC to perform quantum teleportation. We further describe an example of an interaction graph for which teleportation gives a logarithmic speedup in the worst-case routing time over swap-based routing. We also study limits on the speedup afforded by quantum teleportation - showing an O(NlogN\−\−\−\−\−\−\−\√) upper bound on the separation in routing time for any interaction graph - and give tighter bounds for some common classes of graphs.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.04185}, url = {https://arxiv.org/abs/2204.04185}, author = {Devulapalli, Dhruv and Schoute, Eddie and Bapat, Aniruddha and Andrew M. Childs and Alexey V. Gorshkov} } @article {2997, title = {Quantum Simulation for High Energy Physics}, year = {2022}, month = {4/7/2022}, abstract = {

It is for the first time that Quantum Simulation for High Energy Physics (HEP) is studied in the U.S. decadal particle-physics community planning, and in fact until recently, this was not considered a mainstream topic in the community. This fact speaks of a remarkable rate of growth of this subfield over the past few years, stimulated by the impressive advancements in Quantum Information Sciences (QIS) and associated technologies over the past decade, and the significant investment in this area by the government and private sectors in the U.S. and other countries. High-energy physicists have quickly identified problems of importance to our understanding of nature at the most fundamental level, from tiniest distances to cosmological extents, that are intractable with classical computers but may benefit from quantum advantage. They have initiated, and continue to carry out, a vigorous program in theory, algorithm, and hardware co-design for simulations of relevance to the HEP mission. This community whitepaper is an attempt to bring this exciting and yet challenging area of research to the spotlight, and to elaborate on what the promises, requirements, challenges, and potential solutions are over the next decade and beyond.

}, keywords = {FOS: Physical sciences, High Energy Physics - Lattice (hep-lat), High Energy Physics - Phenomenology (hep-ph), High Energy Physics - Theory (hep-th), Nuclear Theory (nucl-th), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.03381}, url = {https://arxiv.org/abs/2204.03381}, author = {Bauer, Christian W. and Zohreh Davoudi and Balantekin, A. Baha and Bhattacharya, Tanmoy and Carena, Marcela and de Jong, Wibe A. and Draper, Patrick and El-Khadra, Aida and Gemelke, Nate and Hanada, Masanori and Kharzeev, Dmitri and Lamm, Henry and Li, Ying-Ying and Liu, Junyu and Lukin, Mikhail and Meurice, Yannick and Monroe, Christopher and Nachman, Benjamin and Pagano, Guido and Preskill, John and Rinaldi, Enrico and Roggero, Alessandro and Santiago, David I. and Savage, Martin J. and Siddiqi, Irfan and Siopsis, George and Van Zanten, David and Wiebe, Nathan and Yamauchi, Yukari and Yeter-Aydeniz, K{\"u}bra and Zorzetti, Silvia} } @article {3204, title = {A scheme to create and verify scalable entanglement in optical lattice}, journal = {npj Quantum Information}, volume = {8}, year = {2022}, month = {9/4/2022}, abstract = {

To achieve scalable quantum information processing, great efforts have been devoted to the creation of large-scale entangled states in various physical systems. Ultracold atom in optical lattice is considered as one of the promising platforms due to its feasible initialization and parallel manipulation. In this work, we propose an efficient scheme to generate and characterize global entanglement in the optical lattice. With only two-layer quantum circuits, the generation utilizes two-qubit entangling gates based on the superexchange interaction in double wells. The parallelism of these operations enables the generation to be fast and scalable. To verify the entanglement of this non-stabilizer state, we mainly design three complementary detection protocols which are less resource-consuming compared to the full tomography. In particular, one just needs two homogenous local measurement settings to identify the entanglement property. Our entanglement generation and verification protocols provide the foundation for the further quantum information processing in optical lattice.

}, doi = {10.1038/s41534-022-00609-0}, url = {https://arxiv.org/abs/2209.01531}, author = {You Zhou and Bo Xiao and Meng-Da Li and Qi Zhao and Zhen-Sheng Yuan and Xiongfeng Ma and Jian-Wei Pan} } @article {3178, title = {Sharp complexity phase transitions generated by entanglement}, year = {2022}, month = {12/20/2022}, abstract = {

Entanglement is one of the physical properties of quantum systems responsible for the computational hardness of simulating quantum systems. But while the runtime of specific algorithms, notably tensor network algorithms, explicitly depends on the amount of entanglement in the system, it is unknown whether this connection runs deeper and entanglement can also cause inherent, algorithm-independent complexity. In this work, we quantitatively connect the entanglement present in certain quantum systems to the computational complexity of simulating those systems. Moreover, we completely characterize the entanglement and complexity as a function of a system parameter. Specifically, we consider the task of simulating single-qubit measurements of k--regular graph states on n qubits. We show that, as the regularity parameter is increased from 1 to n\−1, there is a sharp transition from an easy regime with low entanglement to a hard regime with high entanglement at k=3, and a transition back to easy and low entanglement at k=n\−3. As a key technical result, we prove a duality for the simulation complexity of regular graph states between low and high regularity.

}, keywords = {Computational Complexity (cs.CC), FOS: Computer and information sciences, FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2212.10582}, url = {https://arxiv.org/abs/2212.10582}, author = {Ghosh, Soumik and Deshpande, Abhinav and Hangleiter, Dominik and Alexey V. Gorshkov and Fefferman, Bill} } @article {3071, title = {Simulation Complexity of Many-Body Localized Systems}, year = {2022}, month = {5/25/2022}, abstract = {

We use complexity theory to rigorously investigate the difficulty of classically simulating evolution under many-body localized (MBL) Hamiltonians. Using the defining feature that MBL systems have a complete set of quasilocal integrals of motion (LIOMs), we demonstrate a transition in the classical complexity of simulating such systems as a function of evolution time. On one side, we construct a quasipolynomial-time tensor-network-inspired algorithm for strong simulation of 1D MBL systems (i.e., calculating the expectation value of arbitrary products of local observables) evolved for any time polynomial in the system size. On the other side, we prove that even weak simulation, i.e. sampling, becomes formally hard after an exponentially long evolution time, assuming widely believed conjectures in complexity theory. Finally, using the consequences of our classical simulation results, we also show that the quantum circuit complexity for MBL systems is sublinear in evolution time. This result is a counterpart to a recent proof that the complexity of random quantum circuits grows linearly in time.\ 

}, url = {https://arxiv.org/abs/2205.12967}, author = {Adam Ehrenberg and Abhinav Deshpande and Christopher L. Baldwin and Dmitry A. Abanin and Alexey V. Gorshkov} } @article {3000, title = {Snowmass 2021 White Paper: The Windchime Project}, year = {2022}, month = {3/14/2022}, abstract = {

The absence of clear signals from particle dark matter in direct detection experiments motivates new approaches in disparate regions of viable parameter space. In this Snowmass white paper, we outline the Windchime project, a program to build a large array of quantum-enhanced mechanical sensors. The ultimate aim is to build a detector capable of searching for Planck mass-scale dark matter purely through its gravitational coupling to ordinary matter. In the shorter term, we aim to search for a number of other physics targets, especially some ultralight dark matter candidates. Here, we discuss the basic design, open R\&D challenges and opportunities, current experimental efforts, and both short- and long-term physics targets of the Windchime project.

}, keywords = {Cosmology and Nongalactic Astrophysics (astro-ph.CO), FOS: Physical sciences, High Energy Physics - Experiment (hep-ex), High Energy Physics - Phenomenology (hep-ph)}, doi = {10.48550/ARXIV.2203.07242}, url = {https://arxiv.org/abs/2203.07242}, author = {The Windchime Collaboration and Attanasio, Alaina and Bhave, Sunil A. and Blanco, Carlos and Carney, Daniel and Demarteau, Marcel and Elshimy, Bahaa and Febbraro, Michael and Feldman, Matthew A. and Ghosh, Sohitri and Hickin, Abby and Hong, Seongjin and Lang, Rafael F. and Lawrie, Benjamin and Li, Shengchao and Liu, Zhen and Maldonado, Juan P. A. and Marvinney, Claire and Oo, Hein Zay Yar and Pai, Yun-Yi and Pooser, Raphael and Qin, Juehang and Sparmann, Tobias J. and Taylor, Jacob M. and Tian, Hao and Tunnell, Christopher} } @article {3063, title = {Status Report on the Third Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {NIST}, year = {2022}, month = {7/2022}, abstract = {

The National Institute of Standards and Technology is in the process of selecting publickey cryptographic algorithms through a public, competition-like process. The new publickey cryptography standards will specify additional digital signature, public-key encryption, and key-establishment algorithms to augment Federal Information Processing Standard (FIPS) 186-4, Digital Signature Standard (DSS), as well as NIST Special Publication (SP) 800-56A Revision 3, Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B Revision 2, Recommendation for Pair-Wise Key Establishment Using Integer Factorization Cryptography. It is intended that these algorithms will be capable of protecting sensitive information well into the foreseeable future, including after the advent of quantum computers.

This report describes the evaluation and selection process of the NIST Post-Quantum Cryptography Standardization process third-round candidates based on public feedback and internal review. The report summarizes each of the 15 third-round candidate algorithms and identifies those selected for standardization, as well as those that will continue to be evaluated in a fourth round of analysis. The public-key encryption and key-establishment algorithm that will be standardized is CRYSTALS\–KYBER. The digital signatures that will be standardized are CRYSTALS\–Dilithium, FALCON, and SPHINCS+. While there are multiple signature algorithms selected, NIST recommends CRYSTALS\–Dilithium as the primary algorithm to be implemented. In addition, four of the alternate key-establishment candidate algorithms will advance to a fourth round of evaluation: BIKE, Classic McEliece, HQC, and SIKE. These candidates are still being considered for future standardization. NIST will also issue a new Call for Proposals for public-key digital signature algorithms to augment and diversify its signature portfolio.

}, doi = {https://doi.org/10.6028/NIST.IR.8413-upd1}, author = {Gorjan Alagic and Daniel Apon and David Cooper and Quynh Dang and Thinh Dang and John Kelsey and Jacob Lichtinger and Carl Miller and Dustin Moody and Rene Peralta and Ray Perlner and Angela Robinson} } @article {3199, title = {Tailoring three-dimensional topological codes for biased noise}, year = {2022}, month = {11/3/2022}, abstract = {

Tailored topological stabilizer codes in two dimensions have been shown to exhibit high storage threshold error rates and improved subthreshold performance under biased Pauli noise. Three-dimensional (3D) topological codes can allow for several advantages including a transversal implementation of non-Clifford logical gates, single-shot decoding strategies, parallelized decoding in the case of fracton codes as well as construction of fractal lattice codes. Motivated by this, we tailor 3D topological codes for enhanced storage performance under biased Pauli noise. We present Clifford deformations of various 3D topological codes, such that they exhibit a threshold error rate of 50\% under infinitely biased Pauli noise. Our examples include the 3D surface code on the cubic lattice, the 3D surface code on a checkerboard lattice that lends itself to a subsystem code with a single-shot decoder, the 3D color code, as well as fracton models such as the X-cube model, the Sierpinski model and the Haah code. We use the belief propagation with ordered statistics decoder (BP-OSD) to study threshold error rates at finite bias. We also present a rotated layout for the 3D surface code, which uses roughly half the number of physical qubits for the same code distance under appropriate boundary conditions. Imposing coprime periodic dimensions on this rotated layout leads to logical operators of weight O(n) at infinite bias and a corresponding exp[\−O(n)] subthreshold scaling of the logical failure rate, where n is the number of physical qubits in the code. Even though this scaling is unstable due to the existence of logical representations with O(1) low-rate Pauli errors, the number of such representations scales only polynomially for the Clifford-deformed code, leading to an enhanced effective distance.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.02116}, url = {https://arxiv.org/abs/2211.02116}, author = {Huang, Eric and Pesah, Arthur and Chubb, Christopher T. and Vasmer, Michael and Dua, Arpit} } @article {3012, title = {Three-dimensional quantum cellular automata from chiral semion surface topological order and beyond}, year = {2022}, month = {2/10/2022}, abstract = {

We construct a novel three-dimensional quantum cellular automaton (QCA) based on a system with short-range entangled bulk and chiral semion boundary topological order. We argue that either the QCA is nontrivial, i.e. not a finite-depth circuit of local quantum gates, or there exists a two-dimensional commuting projector Hamiltonian realizing the chiral semion topological order (characterized by U(1)2 Chern-Simons theory). Our QCA is obtained by first constructing the Walker-Wang Hamiltonian of a certain premodular tensor category of order four, then condensing the deconfined bulk boson at the level of lattice operators. We show that the resulting Hamiltonian hosts chiral semion surface topological order in the presence of a boundary and can be realized as a non-Pauli stabilizer code on qubits, from which the QCA is defined. The construction is then generalized to a class of QCAs defined by non-Pauli stabilizer codes on 2n-dimensional qudits that feature surface anyons described by U(1)2n Chern-Simons theory. Our results support the conjecture that the group of nontrivial three-dimensional QCAs is isomorphic to the Witt group of non-degenerate braided fusion categories.

}, keywords = {FOS: Physical sciences, Mathematical Physics (math-ph), Quantum Physics (quant-ph), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2202.05442}, url = {https://arxiv.org/abs/2202.05442}, author = {Shirley, Wilbur and Chen, Yu-An and Dua, Arpit and Ellison, Tyler D. and Tantivasadakarn, Nathanan and Williamson, Dominic J.} } @article {2867, title = {Complexity of Fermionic Dissipative Interactions and Applications to Quantum Computing}, journal = {PRX Quantum}, volume = {2}, year = {2021}, month = {9/17/2021}, abstract = {

Interactions between particles are usually a resource for quantum computing, making quantum many-body systems intractable by any known classical algorithm. In contrast, noise is typically considered as being inimical to quantum many-body correlations, ultimately leading the system to a classically tractable state. This work shows that noise represented by two-body processes, such as pair loss, plays the same role as many-body interactions and makes otherwise classically simulable systems universal for quantum computing. We analyze such processes in detail and establish a complexity transition between simulable and nonsimulable systems as a function of a tuning parameter. We determine important classes of simulable and nonsimulable two-body dissipation. Finally, we show how using resonant dissipation in cold atoms can enhance the performance of two-qubit gates.\ 

}, issn = {2691-3399}, doi = {10.1103/prxquantum.2.030350}, url = {http://dx.doi.org/10.1103/PRXQuantum.2.030350}, author = {Shtanko, Oles and Deshpande, Abhinav and Julienne, Paul S. and Alexey V. Gorshkov} } @article {2819, title = {Garc{\'\i}a-Pintos, Hamma, and del Campo Reply}, journal = {Phys. Rev. Lett.}, volume = {127}, year = {2021}, month = {7/9/2021}, pages = {028902}, abstract = {

We acknowledge that a derivation reported in Phys. Rev. Lett. 125, 040601 (2020) is incorrect as pointed out by Cusumano and Rudnicki. We respond by giving a correct proof of the claim \“fluctuations in the free energy operator upper bound the charging power of a quantum battery\” that we made in the Letter.

}, doi = {10.1103/PhysRevLett.127.028902}, author = {Luis Pedro Garc{\'\i}a-Pintos and Alioscia Hamma and Adolfo del Campo} } @article {2934, title = {Limits to Perception by Quantum Monitoring with Finite Efficiency}, journal = {Entropy}, volume = {23}, year = {2021}, month = {11/17/2021}, pages = {1527}, abstract = {

We formulate limits to perception under continuous quantum measurements by comparing the quantum states assigned by agents that have partial access to measurement outcomes. To this end, we provide bounds on the trace distance and the relative entropy between the assigned state and the actual state of the system. These bounds are expressed solely in terms of the purity and von Neumann entropy of the state assigned by the agent, and are shown to characterize how an agent\’s perception of the system is altered by access to additional information. We apply our results to Gaussian states and to the dynamics of a system embedded in an environment illustrated on a quantum Ising chain.

}, issn = {1099-4300}, doi = {10.3390/e23111527}, url = {https://www.mdpi.com/1099-4300/23/11/1527}, author = {Garc{\'\i}a-Pintos, Luis Pedro and del Campo, Adolfo} } @article {2773, title = {Quantum Computational Supremacy via High-Dimensional Gaussian Boson Sampling}, year = {2021}, month = {2/24/2021}, abstract = {

Photonics is a promising platform for demonstrating quantum computational supremacy (QCS) by convincingly outperforming the most powerful classical supercomputers on a well-defined computational task. Despite this promise, existing photonics proposals and demonstrations face significant hurdles. Experimentally, current implementations of Gaussian boson sampling lack programmability or have prohibitive loss rates. Theoretically, there is a comparative lack of rigorous evidence for the classical hardness of GBS. In this work, we make significant progress in improving both the theoretical evidence and experimental prospects. On the theory side, we provide strong evidence for the hardness of Gaussian boson sampling, placing it on par with the strongest theoretical proposals for QCS. On the experimental side, we propose a new QCS architecture, high-dimensional Gaussian boson sampling, which is programmable and can be implemented with low loss rates using few optical components. We show that particular classical algorithms for simulating GBS are vastly outperformed by high-dimensional Gaussian boson sampling experiments at modest system sizes. This work thus opens the path to demonstrating QCS with programmable photonic processors.

}, url = {https://arxiv.org/abs/2102.12474}, author = {Abhinav Deshpande and Arthur Mehta and Trevor Vincent and Nicolas Quesada and Marcel Hinsche and Marios Ioannou and Lars Madsen and Jonathan Lavoie and Haoyu Qi and Jens Eisert and Dominik Hangleiter and Bill Fefferman and Ish Dhand} } @article {2870, title = {Subdiffusive hydrodynamics of nearly-integrable anisotropic spin chains}, year = {2021}, month = {9/27/2021}, abstract = {

We address spin transport in the easy-axis Heisenberg spin chain subject to integrability-breaking perturbations. We find that spin transport is subdiffusive with dynamical exponent z=4 up to a timescale that is parametrically long in the anisotropy. In the limit of infinite anisotropy, transport is subdiffusive at all times; for large finite anisotropy, one eventually recovers diffusion at late times, but with a diffusion constant independent of the strength of the integrability breaking perturbation. We provide numerical evidence for these findings, and explain them by adapting the generalized hydrodynamics framework to nearly integrable dynamics. Our results show that the diffusion constant of near-integrable interacting spin chains is not generically a continuous function of the integrability-breaking parameter.\ 

}, url = {https://arxiv.org/abs/2109.13251}, author = {Jacopo De Nardis and Sarang Gopalakrishnan and Romain Vasseur and Brayden Ware} } @article {2909, title = {Tight bounds on the convergence of noisy random circuits to uniform}, year = {2021}, month = {12/1/2021}, abstract = {

We study the properties of output distributions of noisy, random circuits. We obtain upper and lower bounds on the expected distance of the output distribution from the uniform distribution. These bounds are tight with respect to the dependence on circuit depth. Our proof techniques also allow us to make statements about the presence or absence of anticoncentration for both noisy and noiseless circuits. We uncover a number of interesting consequences for hardness proofs of sampling schemes that aim to show a quantum computational advantage over classical computation. Specifically, we discuss recent barrier results for depth-agnostic and/or noise-agnostic proof techniques. We show that in certain depth regimes, noise-agnostic proof techniques might still work in order to prove an often-conjectured claim in the literature on quantum computational advantage, contrary to what was thought prior to this work.\ 

}, url = {https://arxiv.org/abs/2112.00716}, author = {Abhinav Deshpande and Bill Fefferman and Alexey V. Gorshkov and Michael Gullans and Pradeep Niroula and Oles Shtanko} } @article {2866, title = {Unifying Quantum and Classical Speed Limits on Observables}, year = {2021}, month = {8/9/2021}, abstract = {

The presence of noise or the interaction with an environment can radically change the dynamics of observables of an otherwise isolated quantum system. We derive a bound on the speed with which observables of open quantum systems evolve. This speed limit divides into Mandalestam and Tamm\&$\#$39;s original time-energy uncertainty relation and a time-information uncertainty relation recently derived for classical systems, generalizing both to open quantum systems. By isolating the coherent and incoherent contributions to the system dynamics, we derive both lower and upper bounds to the speed of evolution. We prove that the latter provide tighter limits on the speed of observables than previously known quantum speed limits, and that a preferred basis of \emph{speed operators} serves to completely characterize the observables that saturate the speed limits. We use this construction to bound the effect of incoherent dynamics on the evolution of an observable and to find the Hamiltonian that gives the maximum coherent speedup to the evolution of an observable.

}, url = {https://arxiv.org/abs/2108.04261}, author = {Luis Pedro Garc{\'\i}a-Pintos and Schuyler Nicholson and Jason R. Green and Adolfo del Campo and Alexey V. Gorshkov} } @article {2449, title = {Auto-tuning of double dot devices in situ with machine learning}, journal = {Phys. Rev. Applied }, volume = {13}, year = {2020}, month = {4/1/2020}, abstract = {

There are myriad quantum computing approaches, each having its own set of challenges to understand and effectively control their operation. Electrons confined in arrays of semiconductor nanostructures, called quantum dots (QDs), is one such approach. The easy access to control parameters, fast measurements, long qubit lifetimes, and the potential for scalability make QDs especially attractive. However, as the size of the QD array grows, so does the number of parameters needed for control and thus the tuning complexity. The current practice of manually tuning the qubits is a relatively time-consuming procedure and is inherently impractical for scaling up and applications. In this work, we report on the in situ implementation of an auto-tuning protocol proposed by Kalantre et al. [arXiv:1712.04914]. In particular, we discuss how to establish a seamless communication protocol between a machine learning (ML)-based auto-tuner and the experimental apparatus. We then show that a ML algorithm trained exclusively on synthetic data coming from a physical model to quantitatively classify the state of the QD device, combined with an optimization routine, can be used to replace manual tuning of gate voltages in devices. A success rate of over 85 \% is determined for tuning to a double quantum dot regime when at least one of the plunger gates is initiated sufficiently close to the desired state. Modifications to the training network, fitness function, and optimizer are discussed as a path towards further improvement in the success rate when starting both near and far detuned from the target double dot range.

}, doi = {https://doi.org/10.1103/PhysRevApplied.13.034075}, url = {https://arxiv.org/abs/1909.08030}, author = {Justyna P. Zwolak and Thomas McJunkin and Sandesh S. Kalantre and J. P. Dodson and E. R. MacQuarrie and D. E. Savage and M. G. Lagally and S. N. Coppersmith and Mark A. Eriksson and J. M. Taylor} } @article {2457, title = {Entanglement Bounds on the Performance of Quantum Computing Architectures}, journal = {Phys. Rev. Research}, volume = {2}, year = {2020}, month = {9/22/2020}, abstract = {

There are many possible architectures for future quantum computers that designers will need to choose between. However, the process of evaluating a particular connectivity graph\&$\#$39;s performance as a quantum architecture can be difficult. In this paper, we establish a connection between a quantity known as the isoperimetric number and a lower bound on the time required to create highly entangled states. The metric we propose counts resources based on the use of two-qubit unitary operations, while allowing for arbitrarily fast measurements and classical feedback. We describe how these results can be applied to the evaluation of the hierarchical architecture proposed in Phys. Rev. A 98, 062328 (2018). We also show that the time-complexity bound we place on the creation of highly-entangled states can be saturated up to a multiplicative factor logarithmic in the number of qubits.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.033316}, url = {https://arxiv.org/abs/1908.04802}, author = {Zachary Eldredge and Leo Zhou and Aniruddha Bapat and James R. Garrison and Abhinav Deshpande and Frederic T. Chong and Alexey V. Gorshkov} } @article {2677, title = {Entanglement entropy scaling transition under competing monitoring protocols}, year = {2020}, month = {08/19/2020}, abstract = {

Dissipation generally leads to the decoherence of a quantum state. In contrast, numerous recent proposals have illustrated that dissipation can also be tailored to stabilize many-body entangled quantum states. While the focus of these works has been primarily on engineering the non-equilibrium steady state, we investigate the build-up of entanglement in the quantum trajectories. Specifically, we analyze the competition between two different dissipation channels arising from two incompatible continuous monitoring protocols. The first protocol locks the phase of neighboring sites upon registering a quantum jump, thereby generating a long-range entanglement through the system, while the second one destroys the coherence via dephasing mechanism. By studying the unraveling of stochastic quantum trajectories associated with the continuous monitoring protocols, we present a transition for the scaling of the averaged trajectory entanglement entropies, from critical scaling to area-law behavior. Our work provides novel insights into the occurrence of a measurement-induced phase transition within a continuous monitoring protocol.

}, url = {https://arxiv.org/abs/2008.08619}, author = {Mathias Van Regemortel and Ze-Pei Cian and Alireza Seif and Hossein Dehghani and Mohammad Hafezi} } @article {2686, title = {Fault-Tolerant Operation of a Quantum Error-Correction Code}, year = {2020}, month = {9/24/2020}, abstract = {

Quantum error correction protects fragile quantum information by encoding it in a larger quantum system whose extra degrees of freedom enable the detection and correction of errors. An encoded logical qubit thus carries increased complexity compared to a bare physical qubit. Fault-tolerant protocols contain the spread of errors and are essential for realizing error suppression with an error-corrected logical qubit. Here we experimentally demonstrate fault-tolerant preparation, rotation, error syndrome extraction, and measurement on a logical qubit encoded in the 9-qubit Bacon-Shor code. For the logical qubit, we measure an average fault-tolerant preparation and measurement error of 0.6\% and a transversal Clifford gate with an error of 0.3\% after error correction. The result is an encoded logical qubit whose logical fidelity exceeds the fidelity of the entangling operations used to create it. We compare these operations with non-fault-tolerant protocols capable of generating arbitrary logical states, and observe the expected increase in error. We directly measure the four Bacon-Shor stabilizer generators and are able to detect single qubit Pauli errors. These results show that fault-tolerant quantum systems are currently capable of logical primitives with error rates lower than their constituent parts. With the future addition of intermediate measurements, the full power of scalable quantum error-correction can be achieved.\ 

}, url = {https://arxiv.org/abs/2009.11482}, author = {Laird Egan and Dripto M. Debroy and Crystal Noel and Andrew Risinger and Daiwei Zhu and Debopriyo Biswas and Michael Newman and Muyuan Li and Kenneth R. Brown and Marko Cetina and Christopher Monroe} } @article {2461, title = {Fluctuations in Extractable Work Bound the Charging Power of Quantum Batteries}, journal = {Phys. Rev. Lett. }, volume = {125}, year = {2020}, month = {7/22/2020}, abstract = {

We study the connection between the charging power of quantum batteries and the fluctuations of the stored work. We prove that in order to have a non-zero rate of change of the extractable work, the state ρW of the battery cannot be an eigenstate of a {\textquoteleft}\emph{work operator}\&$\#$39;, defined by F \≡ HW + β\−1log(ρW), where HW is the Hamiltonian of the battery and β is the inverse temperature of a reference thermal bath with respect to which the extractable work is calculated. We do so by proving that fluctuations in the stored work upper bound the charging power of a quantum battery. Our findings also suggest that quantum coherence in the battery enhances the charging process, which we illustrate on a toy model of a heat engine.\ 

}, doi = {10.1103/PhysRevLett.125.040601}, url = {https://arxiv.org/abs/1909.03558}, author = {Luis Pedro Garc{\'\i}a-Pintos and Alioscia Hamma and Adolfo del Campo} } @article {2577, title = {Hierarchy of linear light cones with long-range interactions}, journal = {Physical Review X}, volume = {10}, year = {2020}, month = {5/29/2020}, abstract = {

In quantum many-body systems with local interactions, quantum information and entanglement cannot spread outside of a \"linear light cone,\" which expands at an emergent velocity analogous to the speed of light. Yet most non-relativistic physical systems realized in nature have long-range interactions: two degrees of freedom separated by a distance r interact with potential energy V(r)\∝1/rα. In systems with long-range interactions, we rigorously establish a hierarchy of linear light cones: at the same α, some quantum information processing tasks are constrained by a linear light cone while others are not. In one spatial dimension, commutators of local operators \⟨ψ|[Ox(t),Oy]|ψ\⟩ are negligible in every state |ψ\⟩ when |x\−y|≳vt, where v is finite when α\>3 (Lieb-Robinson light cone); in a typical state |ψ\⟩ drawn from the infinite temperature ensemble, v is finite when α\>52 (Frobenius light cone); in non-interacting systems, v is finite in every state when α\>2 (free light cone). These bounds apply to time-dependent systems and are optimal up to subalgebraic improvements. Our theorems regarding the Lieb-Robinson and free light cones, and their tightness, also generalize to arbitrary dimensions. We discuss the implications of our bounds on the growth of connected correlators and of topological order, the clustering of correlations in gapped systems, and the digital simulation of systems with long-range interactions. In addition, we show that quantum state transfer and many-body quantum chaos are bounded by the Frobenius light cone, and therefore are poorly constrained by all Lieb-Robinson bounds.

}, doi = {https://doi.org/10.1103/PhysRevX.10.031009}, url = {https://arxiv.org/abs/2001.11509}, author = {Minh C. Tran and Chi-Fang Chen and Adam Ehrenberg and Andrew Y. Guo and Abhinav Deshpande and Yifan Hong and Zhe-Xuan Gong and Alexey V. Gorshkov and Andrew Lucas} } @article {2625, title = {Impossibility of Quantum Virtual Black-Box Obfuscation of Classical Circuits}, year = {2020}, month = {5/13/2020}, abstract = {

Virtual black-box obfuscation is a strong cryptographic primitive: it encrypts a circuit while maintaining its full input/output functionality. A remarkable result by Barak et al. (Crypto 2001) shows that a general obfuscator that obfuscates classical circuits into classical circuits cannot exist. A promising direction that circumvents this impossibility result is to obfuscate classical circuits into quantum states, which would potentially be better capable of hiding information about the obfuscated circuit. We show that, under the assumption that learning-with-errors (LWE) is hard for quantum computers, this quantum variant of virtual black-box obfuscation of classical circuits is generally impossible. On the way, we show that under the presence of dependent classical auxiliary input, even the small class of classical point functions cannot be quantum virtual black-box obfuscated.

}, url = {https://arxiv.org/abs/2005.06432}, author = {Gorjan Alagic and Zvika Brakerski and Yfke Dulek and Christian Schaffner} } @article {2593, title = {Limits on Classical Simulation of Free Fermions with Dissipation}, year = {2020}, month = {5/21/2020}, abstract = {

Free-fermionic systems are a valuable, but limited, class of many-body problems efficiently simulable on a classical computer. We examine how classical simulability of noninteracting fermions is modified in the presence of Markovian dissipation described by quadratic Lindblad operators, including, for example, incoherent transitions or pair losses. On the one hand, we establish three broad classes of Markovian dynamics that are efficiently simulable classically, by devising efficient algorithms. On the other hand, we demonstrate that, in the worst case, simulating Markovian dynamics with quadratic Lindblad operators is at least as hard as simulating universal quantum circuits. This result is applicable to an experimentally relevant setting in cold atomic systems, where magnetic Feshbach resonances can be used to engineer the desired dissipation. For such systems, our hardness result provides a direct scheme for dissipation-assisted quantum computing with a potential significant advantage in the speed of two-qubit gates and, therefore, in error tolerance.

}, url = {https://arxiv.org/abs/2005.10840}, author = {Oles Shtanko and Abhinav Deshpande and Paul S. Julienne and Alexey V. Gorshkov} } @article {2531, title = {Many-Body Dephasing in a Trapped-Ion Quantum Simulator}, journal = {Phys. Rev. Lett.}, volume = {125}, year = {2020}, month = {8/24/2020}, abstract = {

How a closed interacting quantum many-body system relaxes and dephases as a function of time is a fundamental question in thermodynamic and statistical physics. In this work, we observe and analyse the persistent temporal fluctuations after a quantum quench of a tunable long-range interacting transverse-field Ising Hamiltonian realized with a trapped-ion quantum simulator. We measure the temporal fluctuations in the average magnetization of a finite-size system of spin-1/2 particles and observe the experimental evidence for the theoretically predicted regime of many-body dephasing. We experiment in a regime where the properties of the system are closely related to the integrable Hamiltonian with global spin-spin coupling, which enables analytical predictions even for the long-time non-integrable dynamics. We find that the measured fluctuations are exponentially suppressed with increasing system size, consistent with theoretical predictions.\ 

}, doi = {https://doi.org/10.1103/PhysRevLett.125.120605}, url = {https://arxiv.org/abs/2001.02477}, author = {Harvey B. Kaplan and Lingzhen Guo and Wen Lin Tan and Arinjoy De and Florian Marquardt and Guido Pagano and Christopher Monroe} } @article {2689, title = {Optimal state transfer and entanglement generation in power-law interacting systems}, year = {2020}, month = {10/6/2020}, abstract = {

We present an optimal protocol for encoding an unknown qubit state into a multiqubit Greenberger-Horne-Zeilinger-like state and, consequently, transferring quantum information in large systems exhibiting power-law (1/rα) interactions. For all power-law exponents α between d and 2d+1, where d is the dimension of the system, the protocol yields a polynomial speedup for α\>2d and a superpolynomial speedup for α\≤2d, compared to the state of the art. For all α\>d, the protocol saturates the Lieb-Robinson bounds (up to subpolynomial corrections), thereby establishing the optimality of the protocol and the tightness of the bounds in this regime. The protocol has a wide range of applications, including in quantum sensing, quantum computing, and preparation of topologically ordered states.\ 

}, url = {https://arxiv.org/abs/2010.02930}, author = {Minh C. Tran and Abhinav Deshpande and Andrew Y. Guo and Andrew Lucas and Alexey V. Gorshkov} } @article {2557, title = {Quantum Coupon Collector}, journal = {Proceedings of the 15th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2020), Leibniz International Proceedings in Informatics}, volume = {158}, year = {2020}, month = {2/18/2020}, pages = {10:1-10:17}, abstract = {

We study how efficiently a k-element set S\⊆[n] can be learned from a uniform superposition |S\⟩ of its elements. One can think of |S\⟩=\∑i\∈S|i\⟩/|S|\−\−\−\√ as the quantum version of a uniformly random sample over S, as in the classical analysis of the {\textquoteleft}{\textquoteleft}coupon collector problem.\&$\#$39;\&$\#$39; We show that if k is close to n, then we can learn S using asymptotically fewer quantum samples than random samples. In particular, if there are n\−k=O(1) missing elements then O(k) copies of |S\⟩ suffice, in contrast to the Θ(klogk) random samples needed by a classical coupon collector. On the other hand, if n\−k=Ω(k), then Ω(klogk) quantum samples are~necessary. More generally, we give tight bounds on the number of quantum samples needed for every k and n, and we give efficient quantum learning algorithms. We also give tight bounds in the model where we can additionally reflect through |S\⟩. Finally, we relate coupon collection to a known example separating proper and improper PAC learning that turns out to show no separation in the quantum case.

}, doi = {10.4230/LIPIcs.TQC.2020.10}, url = {https://arxiv.org/abs/2002.07688}, author = {Srinivasan Arunachalam and Aleksandrs Belovs and Andrew M. Childs and Robin Kothari and Ansis Rosmanis and Ronald de Wolf} } @article {2563, title = {Random Quantum Batteries}, journal = {Phys. Rev. Research }, volume = {2}, year = {2020}, month = {5/5/2020}, abstract = {

Quantum nano-devices are fundamental systems in quantum thermodynamics that have been the subject of profound interest in recent years. Among these, quantum batteries play a very important role. In this paper we lay down a theory of random quantum batteries and provide a systematic way of computing the average work and work fluctuations in such devices by investigating their typical behavior. We show that the performance of random quantum batteries exhibits typicality and depends only on the spectral properties of the time evolving operator, the initial state and the measuring Hamiltonian. At given revival times a random quantum battery features a quantum advantage over classical random batteries. Our method is particularly apt to be used both for exactly solvable models like the Jaynes-Cummings model or in perturbation theory, e.g., systems subject to harmonic perturbations. We also study the setting of quantum adiabatic random batteries.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.023095}, url = {https://arxiv.org/abs/1908.08064}, author = {Francesco Caravelli and Ghislaine Coulter-De Wit and Luis Pedro Garc{\'\i}a-Pintos and Alioscia Hamma} } @article {2676, title = {Status Report on the Second Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {NISTIR 8309}, year = {2020}, month = {07/2020}, abstract = {

The National Institute of Standards and Technology is in the process of selecting one or more public-key cryptographic algorithms through a public, competition-like process. The new public-key cryptography standards will specify one or more additional digital signatures, public-key encryption, and key-establishment algorithms to augment Federal Information Processing Standard (FIPS) 186-4, Digital Signature Standard (DSS), as well as NIST Special Publication (SP) 800-56A Revision 3, Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B Revision 2, Recommendation for Pair-Wise Key Establishment Using Integer Factorization Cryptography. It is intended that these algorithms will be capable of protecting sensitive information well into the foreseeable future, including after the advent of quantum computers.

The NIST Post-Quantum Cryptography Standardization Process began in 2017 with 69 candidate algorithms that met both the minimum acceptance criteria and submission requirements. The first round lasted until January 2019, during which candidate algorithms were evaluated based on their security, performance, and other characteristics. NIST selected 26 algorithms to advance to the second round for more analysis. This report describes the evaluation and selection process, based on public feedback and internal review, of the second-round candidates. The report summarizes the 26 second-round candidate algorithms and identifies those selected to move forward to the third round of the competition. The third-round finalist public-key encryption and key-establishment algorithms are Classic McEliece, CRYSTALS-KYBER, NTRU, and SABER. The third-round finalists for digital signatures are CRYSTALS-DILITHIUM, FALCON, and Rainbow. These finalists will be considered for standardization at the end of the third round. In addition, eight alternate candidate algorithms will also advance to the third round: BIKE, FrodoKEM, HQC, NTRU Prime, SIKE, GeMSS, Picnic, and SPHINCS+. These additional candidates are still being considered for standardization, although this is unlikely to occur at the end of the third round. NIST hopes that the announcement of these finalists and additional candidates will serve to focus the cryptographic community\’s attention during the next round.

}, doi = {https://doi.org/10.6028/NIST.IR.8309}, author = {Gorjan Alagic and Jacob Alperin-Sheriff and Daniel Apon and David Cooper and Quynh Dang and John Kelsey and Yi-Kai Liu and Carl Miller and Dustin Moody and Rene Peralta and Ray Perlner and Angela Robinson and Daniel Smith-Tone} } @article {2518, title = {Time-information uncertainty relations in thermodynamics}, journal = {Nat. Phys.}, year = {2020}, month = {09/21/2020}, abstract = {

Physical systems that power motion and create structure in a fixed amount of time dissipate energy and produce entropy. Whether living or synthetic, systems performing these dynamic functions must balance dissipation and speed. Here, we show that rates of energy and entropy exchange are subject to a speed limit -- a time-information uncertainty relation -- imposed by the rates of change in the information content of the system. This uncertainty relation bounds the time that elapses before the change in a thermodynamic quantity has the same magnitude as its initial standard deviation. From this general bound, we establish a family of speed limits for heat, work, entropy production, and entropy flow depending on the experimental constraints on the system. In all of these inequalities, the time scale of transient dynamical fluctuations is universally bounded by the Fisher information. Moreover, they all have a mathematical form that mirrors the Mandelstam-Tamm version of the time-energy uncertainty relation in quantum mechanics. These bounds on the speed of arbitrary observables apply to transient systems away from thermodynamic equilibrium, independent of the physical assumptions about the stochastic dynamics or their function.\ 

}, doi = {https://doi.org/10.1038/s41567-020-0981-y}, url = {https://arxiv.org/abs/2001.05418}, author = {Schuyler B. Nicholson and Luis Pedro Garc{\'\i}a-Pintos and Adolfo del Campo and Jason R. Green} } @article {2453, title = {Towards analog quantum simulations of lattice gauge theories with trapped ions}, journal = {Physical Review Research }, volume = {2}, year = {2020}, month = {4/8/2020}, abstract = {

Gauge field theories play a central role in modern physics and are at the heart of the Standard Model of elementary particles and interactions. Despite significant progress in applying classical computational techniques to simulate gauge theories, it has remained a challenging task to compute the real-time dynamics of systems described by gauge theories. An exciting possibility that has been explored in recent years is the use of highly-controlled quantum systems to simulate, in an analog fashion, properties of a target system whose dynamics are difficult to compute. Engineered atom-laser interactions in a linear crystal of trapped ions offer a wide range of possibilities for quantum simulations of complex physical systems. Here, we devise practical proposals for analog simulation of simple lattice gauge theories whose dynamics can be mapped onto spin-spin interactions in any dimension. These include 1+1D quantum electrodynamics, 2+1D Abelian Chern-Simons theory coupled to fermions, and 2+1D pure Z2 gauge theory. The scheme proposed, along with the optimization protocol applied, will have applications beyond the examples presented in this work, and will enable scalable analog quantum simulation of Heisenberg spin models in any number of dimensions and with arbitrary interaction strengths.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.023015}, url = {https://arxiv.org/abs/1908.03210}, author = {Zohreh Davoudi and Mohammad Hafezi and Christopher Monroe and Guido Pagano and Alireza Seif and Andrew Shaw} } @article {2406, title = {Complexity phase diagram for interacting and long-range bosonic Hamiltonians}, year = {2019}, month = {06/10/2019}, abstract = {

Recent years have witnessed a growing interest in topics at the intersection of many-body physics and complexity theory. Many-body physics aims to understand and classify emergent behavior of systems with a large number of particles, while complexity theory aims to classify computational problems based on how the time required to solve the problem scales as the problem size becomes large. In this work, we use insights from complexity theory to classify phases in interacting many-body systems. Specifically, we demonstrate a \"complexity phase diagram\" for the Bose-Hubbard model with long-range hopping. This shows how the complexity of simulating time evolution varies according to various parameters appearing in the problem, such as the evolution time, the particle density, and the degree of locality. We find that classification of complexity phases is closely related to upper bounds on the spread of quantum correlations, and protocols to transfer quantum information in a controlled manner. Our work motivates future studies of complexity in many-body systems and its interplay with the associated physical phenomena.\ 

}, url = {https://arxiv.org/abs/1906.04178}, author = {Nishad Maskara and Abhinav Deshpande and Minh C. Tran and Adam Ehrenberg and Bill Fefferman and Alexey V. Gorshkov} } @article {2533, title = {Development of Quantum InterConnects for Next-Generation Information Technologies}, year = {2019}, month = {12/13/2019}, abstract = {

Just as classical information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the interconnect, a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a quantum internet poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on Quantum Interconnects that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry and national laboratories is required. This document is a summary from a U.S. National Science Foundation supported workshop held on 31 October - 1 November 2019 in Alexandria, VA. Attendees were charged to identify the scientific and community needs, opportunities, and significant challenges for quantum interconnects over the next 2-5 years.\ 

}, url = {https://arxiv.org/abs/1912.06642}, author = {David Awschalom and Karl K. Berggren and Hannes Bernien and Sunil Bhave and Lincoln D. Carr and Paul Davids and Sophia E. Economou and Dirk Englund and Andrei Faraon and Marty Fejer and Saikat Guha and Martin V. Gustafsson and Evelyn Hu and Liang Jiang and Jungsang Kim and Boris Korzh and Prem Kumar and Paul G. Kwiat and Marko Lon{\v c}ar and Mikhail D. Lukin and David A. B. Miller and Christopher Monroe and Sae Woo Nam and Prineha Narang and Jason S. Orcutt} } @article {2369, title = {Ground-state energy estimation of the water molecule on a trapped ion quantum computer}, year = {2019}, month = {03/07/2019}, abstract = {

Quantum computing leverages the quantum resources of superposition and entanglement to efficiently solve computational problems considered intractable for classical computers. Examples include calculating molecular and nuclear structure, simulating strongly-interacting electron systems, and modeling aspects of material function. While substantial theoretical advances have been made in mapping these problems to quantum algorithms, there remains a large gap between the resource requirements for solving such problems and the capabilities of currently available quantum hardware. Bridging this gap will require a co-design approach, where the expression of algorithms is developed in conjunction with the hardware itself to optimize execution. Here, we describe a scalable co-design framework for solving chemistry problems on a trapped ion quantum computer, and apply it to compute the ground-state energy of the water molecule. The robust operation of the trapped ion quantum computer yields energy estimates with errors approaching the chemical accuracy, which is the target threshold necessary for predicting the rates of chemical reaction dynamics.

}, url = {https://arxiv.org/abs/1902.10171}, author = {Yunseong Nam and Jwo-Sy Chen and Neal C. Pisenti and Kenneth Wright and Conor Delaney and Dmitri Maslov and Kenneth R. Brown and Stewart Allen and Jason M. Amini and Joel Apisdorf and Kristin M. Beck and Aleksey Blinov and Vandiver Chaplin and Mika Chmielewski and Coleman Collins and Shantanu Debnath and Andrew M. Ducore and Kai M. Hudek and Matthew Keesan and Sarah M. Kreikemeier and Jonathan Mizrahi and Phil Solomon and Mike Williams and Jaime David Wong-Campos and Christopher Monroe and Jungsang Kim} } @article {2459, title = {Interference of Temporally Distinguishable Photons Using Frequency-Resolved Detection}, journal = {Phys. Rev. Lett. }, volume = {123}, year = {2019}, month = {9/24/2019}, abstract = {

We demonstrate quantum interference of three photons that are distinguishable in time, by resolving them in the conjugate parameter, frequency. We show that the multiphoton interference pattern in our setup can be manipulated by tuning the relative delays between the photons, without the need for reconfiguring the optical network. Furthermore, we observe that the symmetries of our optical network and the spectral amplitude of the input photons are manifested in the interference pattern. Moreover, we demonstrate time-reversed HOM-like interference in the spectral correlations using time-bin entangled photon pairs. By adding a time-varying dispersion using a phase modulator, our setup can be used to realize dynamically reconfigurable and scalable boson sampling in the time domain as well as frequency-resolved multiboson correlation sampling.

}, doi = {https://doi.org/10.1103/PhysRevLett.123.123603}, url = {https://arxiv.org/abs/1904.03222}, author = {Venkata Vikram Orre and Elizabeth A. Goldschmidt and Abhinav Deshpande and Alexey V. Gorshkov and Vincenzo Tamma and Mohammad Hafezi and Sunil Mittal} } @article {2520, title = {Observation of Domain Wall Confinement and Dynamics in a Quantum Simulator}, year = {2019}, month = {12/23/2019}, abstract = {

Confinement is a ubiquitous mechanism in nature, whereby particles feel an attractive force that increases without bound as they separate. A prominent example is color confinement in particle physics, in which baryons and mesons are produced by quark confinement. Analogously, confinement can also occur in low-energy quantum many-body systems when elementary excitations are confined into bound quasiparticles. Here, we report the first observation of magnetic domain wall confinement in interacting spin chains with a trapped-ion quantum simulator. By measuring how correlations spread, we show that confinement can dramatically suppress information propagation and thermalization in such many-body systems. We are able to quantitatively determine the excitation energy of domain wall bound states from non-equilibrium quench dynamics. Furthermore, we study the number of domain wall excitations created for different quench parameters, in a regime that is difficult to model with classical computers. This work demonstrates the capability of quantum simulators for investigating exotic high-energy physics phenomena, such as quark collision and string breaking

}, url = {https://arxiv.org/abs/1912.11117}, author = {W. L. Tan and P. Becker and F. Liu and G. Pagano and K. S. Collins and A. De and L. Feng and H. B. Kaplan and A. Kyprianidis and R. Lundgren and W. Morong and S. Whitsitt and Alexey V. Gorshkov and C. Monroe} } @article {2362, title = {Opportunities for Nuclear Physics \& Quantum Information Science}, year = {2019}, month = {03/13/2019}, abstract = {

his whitepaper is an outcome of the workshop Intersections between Nuclear Physics and Quantum Information held at Argonne National Laboratory on 28-30 March 2018 [www.phy.anl.gov/npqi2018/]. The workshop brought together 116 national and international experts in nuclear physics and quantum information science to explore opportunities for the two fields to collaborate on topics of interest to the U.S. Department of Energy (DOE) Office of Science, Office of Nuclear Physics, and more broadly to U.S. society and industry. The workshop consisted of 22 invited and 10 contributed talks, as well as three panel discussion sessions. Topics discussed included quantum computation, quantum simulation, quantum sensing, nuclear physics detectors, nuclear many-body problem, entanglement at collider energies, and lattice gauge theories.

}, url = {https://arxiv.org/abs/1903.05453}, author = {I. C. Clo{\"e}t and Matthew R. Dietrich and John Arrington and Alexei Bazavov and Michael Bishof and Adam Freese and Alexey V. Gorshkov and Anna Grassellino and Kawtar Hafidi and Zubin Jacob and Michael McGuigan and Yannick Meurice and Zein-Eddine Meziani and Peter Mueller and Christine Muschik and James Osborn and Matthew Otten and Peter Petreczky and Tomas Polakovic and Alan Poon and Raphael Pooser and Alessandro Roggero and Mark Saffman and Brent VanDevender and Jiehang Zhang and Erez Zohar} } @article {2390, title = {Photon pair condensation by engineered dissipation}, journal = {Phys. Rev. Lett. }, volume = {123}, year = {2019}, month = {04/02/2019}, abstract = {

Dissipation can usually induce detrimental decoherence in a quantum system. However, engineered dissipation can be used to prepare and stabilize coherent quantum many-body states. Here, we show that by engineering dissipators containing photon pair operators, one can stabilize an exotic dark state, which is a condensate of photon pairs with a phase-nematic order. In this system, the usual superfluid order parameter, i.e. single-photon correlation, is absent, while the photon pair correlation exhibits long-range order. Although the dark state is not unique due to multiple parity sectors, we devise an additional type of dissipators to stabilize the dark state in a particular parity sector via a diffusive annihilation process which obeys Glauber dynamics in an Ising model. Furthermore, we propose an implementation of these photon-pair dissipators in circuit-QED architecture.\ 

}, doi = {10.1103/PhysRevLett.123.063602}, url = {https://arxiv.org/abs/1904.00016}, author = {Ze-Pei Cian and Guanyu Zhu and Su-Kuan Chu and Alireza Seif and Wade DeGottardi and Liang Jiang and Mohammad Hafezi} } @article {2415, title = {Polynomial Time Algorithms for Estimating Spectra of Adiabatic Hamiltonians}, journal = {Phys. Rev. A}, volume = {100}, year = {2019}, month = {10/1/2020}, abstract = {

Much research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians with Hamming symmetric potentials, such as the well studied \"spike\" example. Due to the large amount of symmetry in these potentials such problems are readily open to analysis both analytically and computationally. However, more realistic potentials do not have such a high degree of symmetry and may have many local minima. Here we present a somewhat more realistic class of problems consisting of many individually Hamming symmetric potential wells. For two or three such wells we demonstrate that such a problem can be solved exactly in time polynomial in the number of qubits and wells. For greater than three wells, we present a tight binding approach with which to efficiently analyze the performance of such Hamiltonians in an adiabatic computation. We provide several basic examples designed to highlight the usefulness of this toy model and to give insight into using the tight binding approach to examining it, including: (1) adiabatic unstructured search with a transverse field driver and a prior guess to the marked item and (2) a scheme for adiabatically simulating the ground states of small collections of strongly interacting spins, with an explicit demonstration for an Ising model Hamiltonian.

}, doi = {https://doi.org/10.1103/PhysRevA.100.032336}, url = {https://arxiv.org/abs/1905.07461}, author = {Jacob Bringewatt and William Dorland and Stephen P. Jordan} } @article {2529, title = {Programmable Quantum Simulations of Spin Systems with Trapped Ions}, year = {2019}, month = {12/17/2019}, abstract = {

Laser-cooled and trapped atomic ions form an ideal standard for the simulation of interacting quantum spin models. Effective spins are represented by appropriate internal energy levels within each ion, and the spins can be measured with near-perfect efficiency using state-dependent fluorescence techniques. By applying optical fields that exert optical dipole forces on the ions, their Coulomb interaction can be modulated in ways that give rise to long-range and tunable spin-spin interactions that can be reconfigured by shaping the spectrum and pattern of the laser fields. Here we review the theoretical mapping of atomic ions to interacting spin systems, the experimental preparation of complex equilibrium states, and the study of dynamical processes of this many-body interacting quantum system. The use of such quantum simulators for studying spin models may inform our understanding of exotic quantum materials and shed light on interacting quantum systems that cannot be modeled with conventional computers.\ 

}, url = {https://arxiv.org/abs/1912.07845}, author = {C. Monroe and W. C. Campbell and L. -M. Duan and Z. -X. Gong and Alexey V. Gorshkov and P. Hess and R. Islam and K. Kim and G. Pagano and P. Richerme and C. Senko and N. Y. Yao} } @article {2410, title = {Quantum Approximate Optimization with a Trapped-Ion Quantum Simulator}, year = {2019}, month = {06/06/2019}, abstract = {

Quantum computers and simulators may offer significant advantages over their classical counterparts, providing insights into quantum many-body systems and possibly solving exponentially hard problems, such as optimization and satisfiability. Here we report the first implementation of a shallow-depth Quantum Approximate Optimization Algorithm (QAOA) using an analog quantum simulator to estimate the ground state energy of the transverse field Ising model with tunable long-range interactions. First, we exhaustively search the variational control parameters to approximate the ground state energy with up to 40 trapped-ion qubits. We then interface the quantum simulator with a classical algorithm to more efficiently find the optimal set of parameters that minimizes the resulting energy of the system. We finally sample from the full probability distribution of the QAOA output with single-shot and efficient measurements of every qubit.\ 

}, url = {https://arxiv.org/abs/1906.02700}, author = {G. Pagano and A. Bapat and P. Becker and K. S. Collins and A. De and P. W. Hess and H. B. Kaplan and A. Kyprianidis and W. L. Tan and Christopher L. Baldwin and L. T. Brady and A. Deshpande and F. Liu and S. Jordan and Alexey V. Gorshkov and C. Monroe} } @article {2530, title = {Quantum Computer Systems for Scientific Discovery}, year = {2019}, month = {12/16/2019}, abstract = {

The great promise of quantum computers comes with the dual challenges of building them and finding their useful applications. We argue that these two challenges should be considered together, by co-designing full stack quantum computer systems along with their applications in order to hasten their development and potential for scientific discovery. In this context, we identify scientific and community needs, opportunities, and significant challenges for the development of quantum computers for science over the next 2-10 years. This document is written by a community of university, national laboratory, and industrial researchers in the field of Quantum Information Science and Technology, and is based on a summary from a U.S. National Science Foundation workshop on Quantum Computing held on October 21-22, 2019 in Alexandria, VA.

}, url = {https://arxiv.org/abs/1912.07577}, author = {Yuri Alexeev and Dave Bacon and Kenneth R. Brown and Robert Calderbank and Lincoln D. Carr and Frederic T. Chong and Brian DeMarco and Dirk Englund and Edward Farhi and Bill Fefferman and Alexey V. Gorshkov and Andrew Houck and Jungsang Kim and Shelby Kimmel and Michael Lange and Seth Lloyd and Mikhail D. Lukin and Dmitri Maslov and Peter Maunz and Christopher Monroe and John Preskill and Martin Roetteler and Martin Savage and Jeff Thompson and Umesh Vazirani} } @article {2532, title = {Quantum Simulators: Architectures and Opportunities}, year = {2019}, month = {12/14/2019}, abstract = {

Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on \"Programmable Quantum Simulators,\" that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education.\ 

}, url = {https://arxiv.org/abs/1912.06938}, author = {Ehud Altman and Kenneth R. Brown and Giuseppe Carleo and Lincoln D. Carr and Eugene Demler and Cheng Chin and Brian DeMarco and Sophia E. Economou and Mark A. Eriksson and Kai-Mei C. Fu and Markus Greiner and Kaden R. A. Hazzard and Randall G. Hulet and Alicia J. Koll{\'a}r and Benjamin L. Lev and Mikhail D. Lukin and Ruichao Ma and Xiao Mi and Shashank Misra and Christopher Monroe and Kater Murch and Zaira Nazario and Kang-Kuen Ni and Andrew C. Potter and Pedram Roushan} } @article {2418, title = {Resource theory of entanglement for bipartite quantum channels}, year = {2019}, month = {07/08/2019}, abstract = {

The traditional perspective in quantum resource theories concerns how to use free operations to convert one resourceful quantum state to another one. For example, a fundamental and well known question in entanglement theory is to determine the distillable entanglement of a bipartite state, which is equal to the maximum rate at which fresh Bell states can be distilled from many copies of a given bipartite state by employing local operations and classical communication for free. It is the aim of this paper to take this kind of question to the next level, with the main question being: What is the best way of using free channels to convert one resourceful quantum channel to another? Here we focus on the the resource theory of entanglement for bipartite channels and establish several fundamental tasks and results regarding it. In particular, we establish bounds on several pertinent information processing tasks in channel entanglement theory, and we define several entanglement measures for bipartite channels, including the logarithmic negativity and the κ-entanglement. We also show that the max-Rains information of [B{\"a}uml et al., Physical Review Letters, 121, 250504 (2018)] has a divergence interpretation, which is helpful for simplifying the results of this earlier work.\ 

}, url = {https://arxiv.org/abs/1907.04181}, author = {Stefan B{\"a}uml and Siddhartha Das and Xin Wang and Mark M. Wilde} } @article {2624, title = {Status Report on the First Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {School: National Institute for Standards and Technology }, year = {2019}, type = {techreport}, abstract = {

The National Institute of Standards and Technology is in the process of selecting one or more
public-key cryptographic algorithms through a public competition-like process. The new publickey cryptography standards will specify one or more additional digital signature, public-key
encryption, and key-establishment algorithms to augment FIPS 186-4, Digital Signature Standard
(DSS), as well as special publications SP 800-56A Revision 2, Recommendation for Pair-Wise
Key Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B,
Recommendation for Pair-Wise Key-Establishment Schemes Using Integer Factorization. It is
intended that these algorithms will be capable of protecting sensitive information well into the
foreseeable future, including after the advent of quantum computers.
In November 2017, 82 candidate algorithms were submitted to NIST for consideration. Among
these, 69 met both the minimum acceptance criteria and our submission requirements, and were
accepted as First-Round Candidates on Dec. 20, 2017, marking the beginning of the First Round
of the NIST Post-Quantum Cryptography Standardization Process. This report describes the
evaluation criteria and selection process, based on public feedback and internal review of the
first-round candidates, and summarizes the 26 candidate algorithms announced on January 30,
2019 for moving forward to the second round of the competition. The 17 Second-Round
Candidate public-key encryption and key-establishment algorithms are BIKE, Classic McEliece,
CRYSTALS-KYBER, FrodoKEM, HQC, LAC, LEDAcrypt (merger of LEDAkem/LEDApkc),
NewHope, NTRU (merger of NTRUEncrypt/NTRU-HRSS-KEM), NTRU Prime, NTS-KEM,
ROLLO (merger of LAKE/LOCKER/Ouroboros-R), Round5 (merger of Hila5/Round2), RQC,
SABER, SIKE, and Three Bears. The 9 Second-Round Candidates for digital signatures are
CRYSTALS-DILITHIUM, FALCON, GeMSS, LUOV, MQDSS, Picnic, qTESLA, Rainbow,
and SPHINCS+.

}, url = {https://nvlpubs.nist.gov/nistpubs/ir/2019/NIST.IR.8240.pdf}, author = {Gorjan Alagic and J. Alperin-Sheriff and D. Apon and D. Cooper and Q. Dang and Carl Miller and D. Moody and R. Peralta and R. Perlner and A. Robinson and D. Smith-Tone and Yi-Kai Liu} } @article {2412, title = {Two-qubit entangling gates within arbitrarily long chains of trapped ions}, year = {2019}, month = {05/28/2019}, abstract = {

Ion trap systems are a leading platform for large scale quantum computers. Trapped ion qubit crystals are fully-connected and reconfigurable, owing to their long range Coulomb interaction that can be modulated with external optical forces. However, the spectral crowding of collective motional modes could pose a challenge to the control of such interactions for large numbers of qubits. Here, we show that high-fidelity quantum gate operations are still possible with very large trapped ion crystals, simplifying the scaling of ion trap quantum computers. To this end, we present analytical work that determines how parallel entangling gates produce a crosstalk error that falls off as the inverse cube of the distance between the pairs. We also show experimental work demonstrating entangling gates on a fully-connected chain of seventeen 171Yb+ ions with fidelities as high as 97(1)\%.

}, url = {https://arxiv.org/abs/1905.10421}, author = {Kevin A. Landsman and Yukai Wu and Pak Hong Leung and Daiwei Zhu and Norbert M. Linke and Kenneth R. Brown and Luming Duan and Christopher R. Monroe} } @article {2211, title = {Asymmetric Particle Transport and Light-Cone Dynamics Induced by Anyonic Statistics}, journal = {Phys. Rev. Lett}, volume = {121}, year = {2018}, month = {2018/12/20}, abstract = {

We study the non-equilibrium dynamics of Abelian anyons in a one-dimensional system. We find that the interplay of anyonic statistics and interactions gives rise to spatially asymmetric particle transport together with a novel dynamical symmetry that depends on the anyonic statistical angle and the sign of interactions. Moreover, we show that anyonic statistics induces asymmetric spreading of quantum information, characterized by asymmetric light cones of out-of-time-ordered correlators. Such asymmetric dynamics is in sharp contrast with the dynamics of conventional fermions or bosons, where both the transport and information dynamics are spatially symmetric. We further discuss experiments with cold atoms where the predicted phenomena can be observed using state-of-the-art technologies. Our results pave the way toward experimentally probing anyonic statistics through non-equilibrium dynamics.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.250404}, url = {https://arxiv.org/abs/1809.02614}, author = {Fangli Liu and James R. Garrison and Dong-Ling Deng and Zhe-Xuan Gong and Alexey V. Gorshkov} } @article {2047, title = {Diffusion Monte Carlo Versus Adiabatic Computation for Local Hamiltonians}, journal = {Physical Review A}, volume = {97}, year = {2018}, month = {2018/02/15}, pages = {022323}, abstract = {

Most research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians, whose ground states can be expressed with only real, nonnegative amplitudes. This raises the question of whether classical Monte Carlo algorithms can efficiently simulate quantum adiabatic optimization with stoquastic Hamiltonians. Recent results have given counterexamples in which path integral and diffusion Monte Carlo fail to do so. However, most adiabatic optimization algorithms, such as for solving MAX-k-SAT problems, use k-local Hamiltonians, whereas our previous counterexample for diffusion Monte Carlo involved n-body interactions. Here we present a new 6-local counterexample which demonstrates that even for these local Hamiltonians there are cases where diffusion Monte Carlo cannot efficiently simulate quantum adiabatic optimization. Furthermore, we perform empirical testing of diffusion Monte Carlo on a standard well-studied class of permutation-symmetric tunneling problems and similarly find large advantages for quantum optimization over diffusion Monte Carlo.

}, doi = {10.1103/PhysRevA.97.022323}, url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.97.022323}, author = {Jacob Bringewatt and William Dorland and Stephen P. Jordan and Alan Mink} } @article {2210, title = {Dynamical phase transitions in sampling complexity}, journal = {Phys. Rev. Lett.}, volume = {121}, year = {2018}, pages = {12 pages, 4 figures. v3: published version}, abstract = {

We make the case for studying the complexity of approximately simulating (sampling) quantum systems for reasons beyond that of quantum computational supremacy, such as diagnosing phase transitions. We consider the sampling complexity as a function of time\ t\ due to evolution generated by spatially local quadratic bosonic Hamiltonians. We obtain an upper bound on the scaling of\ t\ with the number of bosons\ n\ for which approximate sampling is classically efficient. We also obtain a lower bound on the scaling of\ t\ with\ n\ for which any instance of the boson sampling problem reduces to this problem and hence implies that the problem is hard, assuming the conjectures of Aaronson and Arkhipov [Proc. 43rd Annu. ACM Symp. Theory Comput. STOC \&$\#$39;11]. This establishes a dynamical phase transition in sampling complexity. Further, we show that systems in the Anderson-localized phase are always easy to sample from at arbitrarily long times. We view these results in the light of classifying phases of physical systems based on parameters in the Hamiltonian. In doing so, we combine ideas from mathematical physics and computational complexity to gain insight into the behavior of condensed matter, atomic, molecular and optical systems.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.030501}, url = {https://arxiv.org/abs/1703.05332}, author = {Abhinav Deshpande and Bill Fefferman and Minh C. Tran and Michael Foss-Feig and Alexey V. Gorshkov} } @article {2054, title = {Entanglement of purification: from spin chains to holography}, journal = {Journal of High Energy Physics}, year = {2018}, month = {2018/01/22}, pages = {98}, abstract = {

Purification is a powerful technique in quantum physics whereby a mixed quantum state is extended to a pure state on a larger system. This process is not unique, and in systems composed of many degrees of freedom, one natural purification is the one with minimal entanglement. Here we study the entropy of the minimally entangled purification, called the entanglement of purification, in three model systems: an Ising spin chain, conformal field theories holographically dual to Einstein gravity, and random stabilizer tensor networks. We conjecture values for the entanglement of purification in all these models, and we support our conjectures with a variety of numerical and analytical results. We find that such minimally entangled purifications have a number of applications, from enhancing entanglement-based tensor network methods for describing mixed states to elucidating novel aspects of the emergence of geometry from entanglement in the AdS/CFT correspondence.

}, doi = {10.1007/JHEP01(2018)098}, url = {https://link.springer.com/article/10.1007\%2FJHEP01\%282018\%29098$\#$citeas}, author = {Phuc Nguyen and Trithep Devakul and Matthew G. Halbasch and Michael P. Zaletel and Brian Swingle} } @article {2206, title = {Observation of bound state self-interaction in a nano-eV atom collider}, journal = {Nature Communications }, volume = {9}, year = {2018}, month = {2018/11/20}, abstract = {

Quantum mechanical scattering resonances for colliding particles occur when a continuum scattering state couples to a discrete bound state between them. The coupling also causes the bound state to interact with itself via the continuum and leads to a shift in the bound state energy, but, lacking knowledge of the bare bound state energy, measuring this self-energy via the resonance position has remained elusive. Here, we report on the direct observation of self-interaction by using a nano-eV atom collider to track the position of a magnetically-tunable Feshbach resonance through a parameter space spanned by energy and magnetic field. Our system of potassium and rubidium atoms displays a strongly non-monotonic resonance trajectory with an exceptionally large self-interaction energy arising from an interplay between the Feshbach bound state and a different, virtual bound state at a fixed energy near threshold.

}, doi = {https://doi.org/10.1038/s41467-018-07375-8}, url = {https://arxiv.org/abs/1807.01174}, author = {Ryan Thomas and Matthew Chilcott and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {2259, title = {Optimal Pure-State Qubit Tomography via Sequential Weak Measurements}, journal = {Phys. Rev. Lett. }, volume = {121}, year = {2018}, abstract = {

The spin-coherent-state positive-operator-valued-measure (POVM) is a fundamental measurement in quantum science, with applications including tomography, metrology, teleportation, benchmarking, and measurement of Husimi phase space probabilities. We prove that this POVM is achieved by collectively measuring the spin projection of an ensemble of qubits weakly and isotropically. We apply this in the context of optimal tomography of pure qubits. We show numerically that through a sequence of weak measurements of random directions of the collective spin component, sampled discretely or in a continuous measurement with random controls, one can approach the optimal bound.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.130404}, url = {https://arxiv.org/abs/1805.01012}, author = {Ezad Shojaee and Christopher S. Jackson and Carlos A. Riofrio and Amir Kalev and Ivan H. Deutsch} } @article {2218, title = {Photon propagation through dissipative Rydberg media at large input rates}, year = {2018}, abstract = {

We study the dissipative propagation of quantized light in interacting Rydberg media under the conditions of electromagnetically induced transparency (EIT). Rydberg blockade physics in optically dense atomic media leads to strong dissipative interactions between single photons. The regime of high incoming photon flux constitutes a challenging many-body dissipative problem. We experimentally study in detail for the first time the pulse shapes and the second-order correlation function of the outgoing field and compare our data with simulations based on two novel theoretical approaches well-suited to treat this many-photon limit. At low incoming flux, we report good agreement between both theories and the experiment. For higher input flux, the intensity of the outgoing light is lower than that obtained from theoretical predictions. We explain this discrepancy using a simple phenomenological model taking into account pollutants, which are nearly-stationary Rydberg excitations coming from the reabsorption of scattered probe photons. At high incoming photon rates, the blockade physics results in unconventional shapes of measured correlation functions.\ 

}, url = {https://arxiv.org/abs/1807.07586}, author = {Przemyslaw Bienias and James Douglas and Asaf Paris-Mandoki and Paraj Titum and Ivan Mirgorodskiy and Christoph Tresp and Emil Zeuthen and Michael Gullans and Marco Manzoni and Sebastian Hofferberth and Darrick Chang and Alexey V. Gorshkov} } @article {2319, title = {Practitioner{\textquoteright}s guide to social network analysis: Examining physics anxiety in an active-learning setting}, year = {2018}, abstract = {

The application of social network analysis (SNA) has recently grown prevalent in science, technology, engineering, and mathematics education research. Research on classroom networks has led to greater understandings of student persistence in physics majors, changes in their career-related beliefs (e.g., physics interest), and their academic success. In this paper, we aim to provide a practitioner\&$\#$39;s guide to carrying out research using SNA, including how to develop data collection instruments, set up protocols for gathering data, as well as identify network methodologies relevant to a wide range of research questions beyond what one might find in a typical primer. We illustrate these techniques using student anxiety data from active-learning physics classrooms. We explore the relationship between students\&$\#$39; physics anxiety and the social networks they participate in throughout the course of a semester. We find that students\&$\#$39; with greater numbers of outgoing interactions are more likely to experience negative anxiety shifts even while we control for {\it pre} anxiety, gender, and final course grade. We also explore the evolution of student networks and find that the second half of the semester is a critical period for participating in interactions associated with decreased physics anxiety. Our study further supports the benefits of dynamic group formation strategies that give students an opportunity to interact with as many peers as possible throughout a semester. To complement our guide to SNA in education research, we also provide a set of tools for letting other researchers use this approach in their work -- the {\it SNA toolbox} -- that can be accessed on GitHub.\ 

}, url = {https://arxiv.org/abs/1809.00337}, author = {Remy Dou and Justyna P. Zwolak} } @article {2248, title = {The quasiprobability behind the out-of-time-ordered correlator}, journal = {Phys. Rev. }, volume = {A}, year = {2018}, month = {04/2018}, chapter = {042105}, abstract = {

Two topics, evolving rapidly in separate fields, were combined recently: The out-of-time-ordered correlator (OTOC) signals quantum-information scrambling in many-body systems. The Kirkwood-Dirac (KD) quasiprobability represents operators in quantum optics. The OTOC has been shown to equal a moment of a summed quasiprobability. That quasiprobability, we argue, is an extension of the KD distribution. We explore the quasiprobability\&$\#$39;s structure from experimental, numerical, and theoretical perspectives. First, we simplify and analyze the weak-measurement and interference protocols for measuring the OTOC and its quasiprobability. We decrease, exponentially in system size, the number of trials required to infer the OTOC from weak measurements. We also construct a circuit for implementing the weak-measurement scheme. Next, we calculate the quasiprobability (after coarse-graining) numerically and analytically: We simulate a transverse-field Ising model first. Then, we calculate the quasiprobability averaged over random circuits, which model chaotic dynamics. The quasiprobability, we find, distinguishes chaotic from integrable regimes. We observe nonclassical behaviors: The quasiprobability typically has negative components. It becomes nonreal in some regimes. The onset of scrambling breaks a symmetry that bifurcates the quasiprobability, as in classical-chaos pitchforks. Finally, we present mathematical properties. The quasiprobability obeys a Bayes-type theorem, for example, that exponentially decreases the memory required to calculate weak values, in certain cases. A time-ordered correlator analogous to the OTOC, insensitive to quantum-information scrambling, depends on a quasiprobability closer to a classical probability. This work not only illuminates the OTOC\&$\#$39;s underpinnings, but also generalizes quasiprobability theory and motivates immediate-future weak-measurement challenges.

}, doi = {https://doi.org/10.1103/PhysRevA.97.042105}, url = {https://arxiv.org/abs/1704.01971}, author = {Nicole Yunger Halpern and Brian Swingle and Justin Dressel} } @article {2507, title = {Thermal management and non-reciprocal control of phonon flow via optomechanics}, journal = {Nat. Commun.}, volume = {9(1)}, year = {2018}, month = {2018/3/23}, abstract = {

Engineering phonon transport in physical systems is a subject of interest in the study of materials and plays a crucial role in controlling energy and heat transfer. Of particular interest are non-reciprocal phononic systems, which in direct analogy to electric diodes, provide a directional flow of energy. Here, we propose an engineered nanostructured material, in which tunable non-reciprocal phonon transport is achieved through optomechanical coupling. Our scheme relies on breaking time-reversal symmetry by a spatially varying laser drive, which manipulates low-energy acoustic phonons. Furthermore, we take advantage of recent developments in the manipulation of high-energy phonons through controlled scattering mechanisms, such as using alloys and introducing disorder. These combined approaches allow us to design an acoustic isolator and a thermal diode. Our proposed device will have potential impact in phonon-based information processing, and heat management in low temperatures.\ 

}, doi = {https://doi.org/10.1038/s41467-018-03624-y}, url = {https://arxiv.org/abs/1710.08967}, author = {Alireza Seif and Wade DeGottardi and Keivan Esfarjani and Mohammad Hafezi} } @article {2215, title = {Unitary Entanglement Construction in Hierarchical Networks}, year = {2018}, abstract = {

The construction of large-scale quantum computers will require modular architectures that allow physical resources to be localized in easy-to-manage packages. In this work, we examine the impact of different graph structures on the preparation of entangled states. We begin by explaining a formal framework, the hierarchical product, in which modular graphs can be easily constructed. This framework naturally leads us to suggest a class of graphs, which we dub hierarchies. We argue that such graphs have favorable properties for quantum information processing, such as a small diameter and small total edge weight, and use the concept of Pareto efficiency to identify promising quantum graph architectures. We present numerical and analytical results on the speed at which large entangled states can be created on nearest-neighbor grids and hierarchy graphs. We also present a scheme for performing circuit placement--the translation from circuit diagrams to machine qubits--on quantum systems whose connectivity is described by hierarchies.

}, url = {https://arxiv.org/abs/1808.07876}, author = {Aniruddha Bapat and Zachary Eldredge and James R. Garrison and Abhinav Desphande and Frederic T. Chong and Alexey V. Gorshkov} } @article {2000, title = {Above threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider}, journal = {Nature Communications}, volume = {8}, year = {2017}, month = {2017/09/06}, abstract = {

Studies of magnetically tunable Feshbach resonances in ultracold atomic gases have predominantly been carried out in the zero collision-energy limit. Here, we explore above threshold collisions at well-defined energies in the vicinity of a narrow magnetic Feshbach resonance by means of a laser-based collider. Our experiment focuses on collisions between ground-state 87Rb atoms in the |F = 2,mF = 0i and |F = 1,mF = 1i hyperfine states, which have a known s-wave resonance at 9.040(7) G at threshold that strongly couples to inelastic channels, where 1 G = 10\−4 T. Using our collider we can track the magnetic field shift in resonance position as the energy is tuned. This presents a challenge due to the narrow width of the resonance in conjunction with inherent broadening mechanisms of the collider. We find, however, that the narrow Feshbach scattering feature becomes imprinted on the spatial distribution of atoms in a fashion that allows for an accurate determination of resonance position as a function of collision energy through a shift in center-of-mass position of the outgoing clouds. This shift has a dispersive line shape with a zero value at the resonance position. We obtain excellent agreement with theory on the resonance position.

}, doi = {10.1038/s41467-017-00458-y}, url = {https://arxiv.org/abs/1704.07109}, author = {Milena S. J. Horvath and Ryan Thomas and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {2267, title = {Advances in Quantum Reinforcement Learning}, journal = {IEEE SMC, Banff, AB}, year = {2017}, month = {2017}, pages = {282-287}, abstract = {

In recent times, there has been much interest in quantum enhancements of machine learning, specifically in the context of data mining and analysis. Reinforcement learning, an interactive form of learning, is, in turn, vital in artificial intelligence-type applications. Also in this case, quantum mechanics was shown to be useful, in certain instances. Here, we elucidate these results, and show that quantum enhancements can be achieved in a new setting: the setting of learning models which learn how to improve themselves -- that is, those that meta-learn. While not all learning models meta-learn, all non-trivial models have the potential of being \"lifted\", enhanced, to meta-learning models. Our results show that also such models can be quantum-enhanced to make even better learners. In parallel, we address one of the bottlenecks of current quantum reinforcement learning approaches: the need for so-called oracularized variants of task environments. Here we elaborate on a method which realizes these variants, with minimal changes in the setting, and with no corruption of the operative specification of the environments. This result may be important in near-term experimental demonstrations of quantum reinforcement learning.

}, doi = {https://doi.org/10.1109/SMC.2017.8122616}, url = {https://arxiv.org/abs/1811.08676}, author = {Vedran Dunjko and J. M. Taylor and Hans J. Briegel} } @article {1971, title = {Complete 3-Qubit Grover Search on a Programmable Quantum Computer}, journal = {Nature Communications, accepted}, year = {2017}, month = {2017/03/30}, abstract = {

Searching large databases is an important problem with broad applications. The Grover search algorithm provides a powerful method for quantum computers to perform searches with a quadratic speedup in the number of required database queries over classical computers. It is an optimal search algorithm for a quantum computer, and has further applications as a subroutine for other quantum algorithms. Searches with two qubits have been demonstrated on a variety of platforms and proposed for others, but larger search spaces have only been demonstrated on a non-scalable NMR system. Here, we report results for a complete three-qubit Grover search algorithm using the scalable quantum computing technology of trapped atomic ions, with better-than-classical performance. The algorithm is performed for all 8 possible single-result oracles and all 28 possible two-result oracles. Two methods of state marking are used for the oracles: a phase-flip method employed by other experimental demonstrations, and a Boolean method requiring an ancilla qubit that is directly equivalent to the state-marking scheme required to perform a classical search. All quantum solutions are shown to outperform their classical counterparts. We also report the first implementation of a Toffoli-4 gate, which is used along with Toffoli-3 gates to construct the algorithms; these gates have process fidelities of 70.5\% and 89.6\%, respectively.

}, url = {https://arxiv.org/abs/1703.10535}, author = {C. Figgatt and Dmitri Maslov and K. A. Landsman and N. M. Linke and S. Debnath and Christopher Monroe} } @article {1952, title = {Complexity of sampling as an order parameter}, year = {2017}, month = {2017/03/15}, abstract = {

We consider the classical complexity of approximately simulating time evolution under spatially local quadratic bosonic Hamiltonians for time\ t. We obtain upper and lower bounds on the scaling of\ twith the number of bosons,\ n, for which simulation, cast as a sampling problem, is classically efficient and provably hard, respectively. We view these results in the light of classifying phases of physical systems based on parameters in the Hamiltonian and conjecture a link to dynamical phase transitions. In doing so, we combine ideas from mathematical physics and computational complexity to gain insight into the behavior of condensed matter systems.

}, url = {https://arxiv.org/abs/1703.05332}, author = {Abhinav Deshpande and Bill Fefferman and Michael Foss-Feig and Alexey V. Gorshkov} } @article {1959, title = {Dispersive optical detection of magnetic Feshbach resonances in ultracold gases}, journal = {Physical Review A}, volume = {96}, year = {2017}, month = {2017/08/18}, pages = {022705}, abstract = {

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of\ 87Rb, between the\ |F=1,mF=1\⟩\ and\ |F=2,mF=0\⟩\ states. Despite the loss features being\ 100\ mG wide, we require only 21 experimental runs to explore a magnetic field range \>18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G and two previously unobserved p-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

}, doi = {10.1103/PhysRevA.96.022705}, url = {https://arxiv.org/abs/1702.02216}, author = {Bianca J. Sawyer and Milena S. J. Horvath and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {2059, title = {Efimov States of Strongly Interacting Photons}, journal = {Physical Review Letters}, volume = {119}, year = {2017}, month = {2017/12/04}, pages = {233601}, abstract = {

We demonstrate the emergence of universal Efimov physics for interacting photons in cold gases of Rydberg atoms. We consider the behavior of three photons injected into the gas in their propagating frame, where a paraxial approximation allows us to consider them as massive particles. In contrast to atoms and nuclei, the photons have a large anisotropy between their longitudinal mass, arising from dispersion, and their transverse mass, arising from diffraction. Nevertheless, we show that in suitably rescaled coordinates the effective interactions become dominated by s-wave scattering near threshold and, as a result, give rise to an Efimov effect near unitarity, but with spatially anisotropic wavefunctions in the original coordinates. We show that the three-body loss of these Efimov trimers can be strongly suppressed and determine conditions under which these states are observable in current experiments. These effects can be naturally extended to probe few-body universality beyond three bodies, as well as the role of Efimov physics in the non-equilbrium, many-body regime.

}, doi = {10.1103/PhysRevLett.119.233601}, url = {https://arxiv.org/abs/1709.01955}, author = {Michael Gullans and S. Diehl and S. T. Rittenhouse and B. P. Ruzic and J. P. D{\textquoteright}Incao and P. Julienne and Alexey V. Gorshkov and J. M. Taylor} } @proceedings {1935, title = {Experimental Comparison of Two Quantum Computing Architectures}, volume = {114}, year = {2017}, month = {2017/03/21}, pages = {3305-3310}, edition = {13}, abstract = {

We run a selection of algorithms on two state-of-the-art 5-qubit quantum computers that are based on different technology platforms. One is a publicly accessible superconducting transmon device [1] with limited connectivity, and the other is a fully connected trapped-ion system [2]. Even though the two systems have different native quantum interactions, both can be programmed in a way that is blind to the underlying hardware, thus allowing the first comparison of identical quantum algorithms between different physical systems. We show that quantum algorithms and circuits that employ more connectivity clearly benefit from a better connected system of qubits. While the quantum systems here are not yet large enough to eclipse classical computers, this experiment exposes critical factors of scaling quantum computers, such as qubit connectivity and gate expressivity. In addition, the results suggest that co-designing particular quantum applications with the hardware itself will be paramount in successfully using quantum computers in the future.

}, doi = {10.1073/pnas.1618020114}, url = {http://www.pnas.org/content/114/13/3305}, author = {N.M. Linke and Dmitri Maslov and Martin Roetteler and S. Debnath and C. Figgatt and K. A. Landsman and K. Wright and Christopher Monroe} } @article {2130, title = {Experimental Study of Optimal Measurements for Quantum State Tomography}, journal = {Physical Review Letters}, volume = {119}, year = {2017}, month = {2017/10/13}, pages = {150401}, abstract = {

Quantum tomography is a critically important tool to evaluate quantum hardware, making it essential to develop optimized measurement strategies that are both accurate and efficient. We compare a variety of strategies using nearly pure test states. Those that are informationally complete for all states are found to be accurate and reliable even in the presence of errors in the measurements themselves, while those designed to be complete only for pure states are far more efficient but highly sensitive to such errors. Our results highlight the unavoidable trade-offs inherent in quantum tomography.

}, doi = {10.1103/PhysRevLett.119.150401}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.119.150401}, author = {Sosa-Martinez, H. and Lysne, N. K. and Baldwin, C. H. and Kalev, A. and Deutsch, I. H. and Jessen, P. S.} } @article {2201, title = {Exponential improvements for quantum-accessible reinforcement learning}, year = {2017}, abstract = {

Quantum computers can offer dramatic improvements over classical devices for data analysis tasks such as prediction and classification. However, less is known about the advantages that quantum computers may bring in the setting of reinforcement learning, where learning is achieved via interaction with a task environment. Here, we consider a special case of reinforcement learning, where the task environment allows quantum access. In addition, we impose certain \"naturalness\" conditions on the task environment, which rule out the kinds of oracle problems that are studied in quantum query complexity (and for which quantum speedups are well-known). Within this framework of quantum-accessible reinforcement learning environments, we demonstrate that quantum agents can achieve exponential improvements in learning efficiency, surpassing previous results that showed only quadratic improvements. A key step in the proof is to construct task environments that encode well-known oracle problems, such as Simon\&$\#$39;s problem and Recursive Fourier Sampling, while satisfying the above \"naturalness\" conditions for reinforcement learning. Our results suggest that quantum agents may perform well in certain game-playing scenarios, where the game has recursive structure, and the agent can learn by playing against itself

}, url = {https://arxiv.org/abs/1710.11160}, author = {Vedran Dunjko and Yi-Kai Liu and Xingyao Wu and J. M. Taylor} } @article {2009, title = {Extreme learning machines for regression based on V-matrix method}, journal = {Cognitive Neurodynamics}, year = {2017}, month = {2017/06/10}, abstract = {

This paper studies the joint effect of V-matrix, a recently proposed framework for statistical inferences, and extreme learning machine (ELM) on regression problems. First of all, a novel algorithm is proposed to efficiently evaluate the V-matrix. Secondly, a novel weighted ELM algorithm called V-ELM is proposed based on the explicit kernel mapping of ELM and the V-matrix method. Though V-matrix method could capture the geometrical structure of training data, it tends to assign a higher weight to instance with smaller input value. In order to avoid this bias, a novel method called VI-ELM is proposed by minimizing both the regression error and the V-matrix weighted error simultaneously. Finally, experiment results on 12 real world benchmark datasets show the effectiveness of our proposed methods.

}, issn = {1871-4099}, doi = {10.1007/s11571-017-9444-2}, url = {http://dx.doi.org/10.1007/s11571-017-9444-2}, author = {Yang, Zhiyong and Zhang, Taohong and Lu, Jingcheng and Yuan Su and Zhang, Dezheng and Duan, Yaowu} } @article {1990, title = {Genuine N -partite entanglement without N -partite correlation functions}, journal = {Physical Review A}, volume = {95}, year = {2017}, month = {2017/06/26}, pages = {062331}, abstract = {

A genuinely\ N-partite entangled state may display vanishing\ N-partite correlations measured for arbitrary local observables. In such states the genuine entanglement is noticeable solely in correlations between subsets of particles. A straightforward way to obtain such states for odd\ N\ is to design an \“antistate\” in which all correlations between an odd number of observers are exactly opposite. Evenly mixing a state with its antistate then produces a mixed state with no\ N-partite correlations, with many of them genuinely multiparty entangled. Intriguingly, all known examples of \“entanglement without correlations\” involve an\ odd\ number of particles. Here we further develop the idea of antistates, thereby shedding light on the different properties of even and odd particle systems. We conjecture that there is no antistate to any pure even-N-party entangled state making the simple construction scheme unfeasible. However, as we prove by construction, higher-rank examples of entanglement without correlations for arbitrary even\ N\ indeed exist. These classes of states exhibit genuine entanglement and even violate an\ N-partite Bell inequality, clearly demonstrating the nonclassical features of these states as well as showing their applicability for quantum information processing.

}, doi = {doi.org/10.1103/PhysRevA.95.062331}, url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.062331}, author = {Minh C. Tran and Margherita Zuppardo and Anna de Rosier and Lukas Knips and Wieslaw Laskowski and Tomasz Paterek and Harald Weinfurter} } @article {2058, title = {Quantum Fully Homomorphic Encryption With Verification}, journal = {Proceedings of ASIACRYPT 2017}, year = {2017}, month = {2017/11/30}, pages = {438-467}, abstract = {

Fully-homomorphic encryption (FHE) enables computation on encrypted data while maintaining secrecy. Recent research has shown that such schemes exist even for quantum computation. Given the numerous applications of classical FHE (zero-knowledge proofs, secure two-party computation, obfuscation, etc.) it is reasonable to hope that quantum FHE (or QFHE) will lead to many new results in the quantum setting. However, a crucial ingredient in almost all applications of FHE is circuit verification. Classically, verification is performed by checking a transcript of the homomorphic computation. Quantumly, this strategy is impossible due to no-cloning. This leads to an important open question: can quantum computations be delegated and verified in a non-interactive manner? In this work, we answer this question in the affirmative, by constructing a scheme for QFHE with verification (vQFHE). Our scheme provides authenticated encryption, and enables arbitrary polynomial-time quantum computations without the need of interaction between client and server. Verification is almost entirely classical; for computations that start and end with classical states, it is completely classical. As a first application, we show how to construct quantum one-time programs from classical one-time programs and vQFHE.

}, doi = {10.1007/978-3-319-70694-8_16}, url = {https://arxiv.org/abs/1708.09156}, author = {Gorjan Alagic and Yfke Dulek and Christian Schaffner and Florian Speelman} } @article {2048, title = {On the readiness of quantum optimization machines for industrial applications}, year = {2017}, month = {2017/08/31}, abstract = {

There have been multiple attempts to demonstrate that quantum annealing and, in particular, quantum annealing on quantum annealing machines, has the potential to outperform current classical optimization algorithms implemented on CMOS technologies. The benchmarking of these devices has been controversial. Initially, random spin-glass problems were used, however, these were quickly shown to be not well suited to detect any quantum speedup. Subsequently, benchmarking shifted to carefully crafted synthetic problems designed to highlight the quantum nature of the hardware while (often) ensuring that classical optimization techniques do not perform well on them. Even worse, to date a true sign of improved scaling with the number problem variables remains elusive when compared to classical optimization techniques. Here, we analyze the readiness of quantum annealing machines for real-world application problems. These are typically not random and have an underlying structure that is hard to capture in synthetic benchmarks, thus posing unexpected challenges for optimization techniques, both classical and quantum alike. We present a comprehensive computational scaling analysis of fault diagnosis in digital circuits, considering architectures beyond D-wave quantum annealers. We find that the instances generated from real data in multiplier circuits are harder than other representative random spin-glass benchmarks with a comparable number of variables. Although our results show that transverse-field quantum annealing is outperformed by state-of-the-art classical optimization algorithms, these benchmark instances are hard and small in the size of the input, therefore representing the first industrial application ideally suited for near-term quantum annealers.

}, url = {https://arxiv.org/abs/1708.09780}, author = {Alejandro Perdomo-Ortiz and Alexander Feldman and Asier Ozaeta and Sergei V. Isakov and Zheng Zhu and Bryan O{\textquoteright}Gorman and Helmut G. Katzgraber and Alexander Diedrich and Hartmut Neven and Johan de Kleer and Brad Lackey and Rupak Biswas} } @article {2099, title = {Super-polynomial and exponential improvements for quantum-enhanced reinforcement learning}, year = {2017}, month = {2017/12/12}, abstract = {

Recent work on quantum machine learning has demonstrated that quantum computers can offer dramatic improvements over classical devices for data mining, prediction and classification. However, less is known about the advantages using quantum computers may bring in the more general setting of reinforcement learning, where learning is achieved via interaction with a task environment that provides occasional rewards. Reinforcement learning can incorporate data-analysis-oriented learning settings as special cases, but also includes more complex situations where, e.g., reinforcing feedback is delayed. In a few recent works, Grover-type amplification has been utilized to construct quantum agents that achieve up-to-quadratic improvements in learning efficiency. These encouraging results have left open the key question of whether super-polynomial improvements in learning times are possible for genuine reinforcement learning problems, that is problems that go beyond the other more restricted learning paradigms. In this work, we provide a family of such genuine reinforcement learning tasks. We construct quantum-enhanced learners which learn super-polynomially, and even exponentially faster than any classical reinforcement learning model, and we discuss the potential impact our results may have on future technologies.

}, url = {https://arxiv.org/abs/1710.11160}, author = {Vedran Dunjko and Yi-Kai Liu and Xingyao Wu and J. M. Taylor} } @article {1915, title = {Demonstration of a small programmable quantum computer with atomic qubits}, journal = {Nature}, volume = {536}, year = {2016}, month = {2016/08/04}, pages = {63-66}, abstract = {

Quantum computers can solve certain problems more efficiently than any possible conventional computer. Small quantum algorithms have been demonstrated on multiple quantum computing platforms, many specifically tailored in hardware to implement a particular algorithm or execute a limited number of computational paths. Here, we demonstrate a five-qubit trapped-ion quantum computer that can be programmed in software to implement arbitrary quantum algorithms by executing any sequence of universal quantum logic gates. We compile algorithms into a fully-connected set of gate operations that are native to the hardware and have a mean fidelity of 98 \%. Reconfiguring these gate sequences provides the flexibility to implement a variety of algorithms without altering the hardware. As examples, we implement the Deutsch-Jozsa (DJ) and Bernstein-Vazirani (BV) algorithms with average success rates of 95 \% and 90 \%, respectively. We also perform a coherent quantum Fourier transform (QFT) on five trappedion qubits for phase estimation and period finding with average fidelities of 62 \% and 84 \%, respectively. This small quantum computer can be scaled to larger numbers of qubits within a single register, and can be further expanded by connecting several such modules through ion shuttling or photonic quantum channels.

}, doi = {10.1038/nature18648}, url = {http://www.nature.com/nature/journal/v536/n7614/full/nature18648.html}, author = {S. Debnath and N. M. Linke and C. Figgatt and K. A. Landsman and K. Wright and C. Monroe} } @article {1910, title = {Experimental demonstration of quantum fault tolerance}, year = {2016}, month = {2016/11/21}, abstract = {

Quantum computers will eventually reach a size at which quantum error correction (QEC) becomes imperative. In order to make quantum information robust to errors introduced by qubit imperfections and flawed control operations, QEC protocols encode a logical qubit in multiple physical qubits. This redundancy allows the extraction of error syndromes and the subsequent correction or detection of errors without destroying the logical state itself through direct measurement. While several experiments have shown a reduction of high intrinsic or artificially introduced errors in logical qubits, fault-tolerant encoding of a logical qubit has never been demonstrated. Here we show the encoding and syndrome measurement of a fault-tolerant logical qubit via an error detection protocol on four physical qubits, represented by trapped atomic ions. This demonstrates for the first time the robustness of a fault-tolerant qubit to imperfections in the very operations used to encode it. This advantage persists in the face of large added error rates and experimental calibration errors.

}, url = {https://arxiv.org/abs/1611.06946}, author = {N. M. Linke and M. Gutierrez and K. A. Landsman and C. Figgatt and S. Debnath and K. R. Brown and C. Monroe} } @article {1789, title = {Lattice Laughlin states on the torus from conformal field theory}, journal = {Journal of Statistical Mechanics: Theory and Experiment}, volume = {2016}, year = {2016}, month = {2016/01/28}, pages = {013102}, abstract = {Conformal field theory has turned out to be a powerful tool to derive two-dimensional lattice models displaying fractional quantum Hall physics. So far most of the work has been for lattices with open boundary conditions in at least one of the two directions, but it is desirable to also be able to handle the case of periodic boundary conditions. Here, we take steps in this direction by deriving analytical expressions for a family of conformal field theory states on the torus that is closely related to the family of bosonic and fermionic Laughlin states. We compute how the states transform when a particle is moved around the torus and when the states are translated or rotated, and we provide numerical evidence in particular cases that the states become orthonormal up to a common factor for large lattices. We use these results to find the S -matrix of the states, which turns out to be the same as for the continuum Laughlin states. Finally, we show that when the states are defined on a square lattice with suitable lattice spacing they practically coincide with the Laughlin states restricted to a lattice.}, url = {http://stacks.iop.org/1742-5468/2016/i=1/a=013102}, author = {Abhinav Deshpande and Anne E B Nielsen} } @article {1914, title = {Multiple scattering dynamics of fermions at an isolated p-wave resonance}, journal = {Nature Communications}, volume = {7}, year = {2016}, month = {2016/07/11}, pages = {12069}, abstract = {

The wavefunction for indistinguishable fermions is anti-symmetric under particle exchange, which directly leads to the Pauli exclusion principle, and hence underlies the structure of atoms and the properties of almost all materials. In the dynamics of collisions between two indistinguishable fermions this requirement strictly prohibits scattering into 90 degree angles. Here we experimentally investigate the collisions of ultracold clouds fermionic\ 40K\ atoms by directly measuring scattering distributions. With increasing collision energy we identify the Wigner threshold for p-wave scattering with its tell-tale dumb-bell shape and no\ 90o\ yield. Above this threshold effects of multiple scattering become manifest as deviations from the underlying binary p-wave shape, adding particles either isotropically or axially. A shape resonance for\ 40K\ facilitates the separate observation of these two processes. The isotropically enhanced multiple scattering mode is a generic p-wave threshold phenomenon, while the axially enhanced mode should occur in any colliding particle system with an elastic scattering resonance.

}, doi = {10.1038/ncomms12069}, url = {http://www.nature.com/articles/ncomms12069}, author = {Ryan Thomas and Kris O. Roberts and Eite Tiesinga and Andrew C.J. Wade and P. Blair Blakie and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {1911, title = {Quantum-Enhanced Machine Learning}, journal = {Physical Review Letters}, volume = {117}, year = {2016}, month = {2016/09/20}, pages = {130501}, abstract = {

The emerging field of quantum machine learning has the potential to substantially aid in the problems and scope of artificial intelligence. This is only enhanced by recent successes in the field of classical machine learning. In this work we propose an approach for the systematic treatment of machine learning, from the perspective of quantum information. Our approach is general and covers all three main branches of machine learning: supervised, unsupervised, and reinforcement learning. While quantum improvements in supervised and unsupervised learning have been reported, reinforcement learning has received much less attention. Within our approach, we tackle the problem of quantum enhancements in reinforcement learning as well, and propose a systematic scheme for providing improvements. As an example, we show that quadratic improvements in learning efficiency, and exponential improvements in performance over limited time periods, can be obtained for a broad class of learning problems.

}, doi = {10.1103/PhysRevLett.117.130501}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.130501}, author = {Dunjko, Vedran and J. M. Taylor and Briegel, Hans J.} } @article {1838, title = {Atom induced cavities and tunable long-range interactions between atoms trapped near photonic crystals}, journal = {Nature Photon. 9, 326 (2015)}, year = {2015}, month = {2015/03/03}, abstract = {

Using cold atoms to simulate strongly interacting quantum systems represents an exciting frontier of physics. However, achieving tunable, coherent long-range interactions between atoms is an outstanding challenge, which currently leaves a large class of models inaccessible to quantum simulation. Here, we propose a solution exploiting the powerful new platform of cold atoms trapped near nano-photonic systems. We show that the dielectric contrast of an atom trapped near a photonic crystal can seed a localized cavity mode around the atomic position. In a dynamic form of \“all-atomic\” cavity QED, the length of these cavity modes can be tuned, and atoms separated by the order of the e\↵ective cavity length can interact coherently with each other. Considering realistic conditions such as fabrication disorder and photon losses, coherent long-range potentials or spin interactions can be dominant in the system over length scales up to hundreds of wavelengths.

}, doi = {doi:10.1038/nphoton.2015.57}, url = {http://www.nature.com/nphoton/journal/v9/n5/full/nphoton.2015.57.html}, author = {J S Douglas and H Habibian and Alexey V. Gorshkov and H J Kimble and D E Chang} } @article {1515, title = {Demonstration of Robust Quantum Gate Tomography via Randomized Benchmarking}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {2015/11/05}, pages = {113019}, abstract = { Typical quantum gate tomography protocols struggle with a self-consistency problem: the gate operation cannot be reconstructed without knowledge of the initial state and final measurement, but such knowledge cannot be obtained without well-characterized gates. A recently proposed technique, known as randomized benchmarking tomography (RBT), sidesteps this self-consistency problem by designing experiments to be insensitive to preparation and measurement imperfections. We implement this proposal in a superconducting qubit system, using a number of experimental improvements including implementing each of the elements of the Clifford group in single {\textquoteleft}atomic{\textquoteright} pulses and custom control hardware to enable large overhead protocols. We show a robust reconstruction of several single-qubit quantum gates, including a unitary outside the Clifford group. We demonstrate that RBT yields physical gate reconstructions that are consistent with fidelities obtained by randomized benchmarking. }, doi = {10.1088/1367-2630/17/11/113019}, url = {http://arxiv.org/abs/1505.06686}, author = {Blake R. Johnson and Marcus P. da Silva and Colm A. Ryan and Shelby Kimmel and Jerry M. Chow and Thomas A. Ohki} } @article {1331, title = {Framework for learning agents in quantum environments}, year = {2015}, month = {2015/07/30}, abstract = {In this paper we provide a broad framework for describing learning agents in general quantum environments. We analyze the types of classically specified environments which allow for quantum enhancements in learning, by contrasting environments to quantum oracles. We show that whether or not quantum improvements are at all possible depends on the internal structure of the quantum environment. If the environments are constructed and the internal structure is appropriately chosen, or if the agent has limited capacities to influence the internal states of the environment, we show that improvements in learning times are possible in a broad range of scenarios. Such scenarios we call luck-favoring settings. The case of constructed environments is particularly relevant for the class of model-based learning agents, where our results imply a near-generic improvement. }, url = {http://arxiv.org/abs/1507.08482v1}, author = {Vedran Dunjko and J. M. Taylor and Hans J. Briegel} } @article {1675, title = {Quantum many-body models with cold atoms coupled to photonic crystals}, journal = {Nature Photonics}, volume = {9}, year = {2015}, month = {2015/04/04}, pages = {326 - 331}, issn = {1749-4885}, doi = {10.1038/nphoton.2015.57}, url = {http://www.nature.com/doifinder/10.1038/nphoton.2015.57}, author = {Douglas, J. S. and Habibian, H. and Hung, C.-L. and Alexey V. Gorshkov and Kimble, H. J. and Chang, D. E.} } @article {1450, title = {Universal Subspaces for Local Unitary Groups of Fermionic Systems}, journal = {Communications in Mathematical Physics}, volume = {333}, year = {2015}, month = {2014/10/10}, pages = {541 - 563}, abstract = { Let $\mathcal{V}=\wedge^N V$ be the $N$-fermion Hilbert space with $M$-dimensional single particle space $V$ and $2N\le M$. We refer to the unitary group $G$ of $V$ as the local unitary (LU) group. We fix an orthonormal (o.n.) basis $\ket{v_1},...,\ket{v_M}$ of $V$. Then the Slater determinants $e_{i_1,...,i_N}:= \ket{v_{i_1}\we v_{i_2}\we...\we v_{i_N}}$ with $i_1<...3. If $M$ is even, the well known BCS states are not LU-equivalent to any single occupancy state. Our main result is that for N=3 and $M$ even there is a universal subspace $\cW\subseteq\cS$ spanned by $M(M-1)(M-5)/6$ states $e_{i_1,...,i_N}$. Moreover the number $M(M-1)(M-5)/6$ is minimal. }, doi = {10.1007/s00220-014-2187-6}, url = {http://arxiv.org/abs/1301.3421v2}, author = {Lin Chen and Jianxin Chen and Dragomir Z. Djokovic and Bei Zeng} } @article {1788, title = {Remote tomography and entanglement swapping via von Neumann{\textendash}Arthurs{\textendash}Kelly interaction }, journal = {Physical Review A}, volume = {89}, year = {2014}, month = {2014/05/09}, pages = {052107}, abstract = {We propose an interaction-based method for remote tomography and entanglement swapping. Alice arranges a von Neumann-Arthurs-Kelly interaction between a system particle P and two apparatus particles A1,A2, and then transports the latter to Bob. Bob can reconstruct the unknown initial state of particle P not received by him by quadrature measurements on A1,A2. Further, if another particle P' in Alice{\textquoteright}s hands is EPR entangled with P, it will be EPR entangled with the distant pair A1,A2. This method will be contrasted with the usual teleportation protocols.}, doi = {http://dx.doi.org/10.1103/PhysRevA.89.052107}, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.89.052107}, author = {S. M. Roy and Abhinav Deshpande and Nitica Sakharwade} } @article {1514, title = {Robust Extraction of Tomographic Information via Randomized Benchmarking}, journal = {Physical Review X}, volume = {4}, year = {2014}, month = {2014/3/25}, abstract = { We describe how randomized benchmarking can be used to reconstruct the unital part of any trace-preserving quantum map, which in turn is sufficient for the full characterization of any unitary evolution, or more generally, any unital trace-preserving evolution. This approach inherits randomized benchmarking{\textquoteright}s robustness to preparation and measurement imperfections, therefore avoiding systematic errors caused by these imperfections. We also extend these techniques to efficiently estimate the average fidelity of a quantum map to unitary maps outside of the Clifford group. The unitaries we consider include operations commonly used to achieve universal quantum computation in a fault-tolerant setting. In addition, we rigorously bound the time and sampling complexities of randomized benchmarking procedures. }, doi = {10.1103/PhysRevX.4.011050}, url = {http://arxiv.org/abs/1306.2348v1}, author = {Shelby Kimmel and Marcus P. da Silva and Colm A. Ryan and Blake R. Johnson and Thomas Ohki} } @article {1486, title = {Individual Addressing in Quantum Computation through Spatial Refocusing}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/11/21}, abstract = { Separate addressing of individual qubits is a challenging requirement for scalable quantum computation, and crosstalk between operations on neighboring qubits remains as a significant source of noise for current experimental implementation of multi-qubit platforms. We propose a scheme based on spatial refocusing from interference of several coherent laser beams to significantly reduce the crosstalk noise for any type of quantum gates. A general framework is developed for the spatial refocusing technique, in particular with practical Gaussian beams, and we show under typical experimental conditions, the crosstalk-induced infidelity of quantum gates can be reduced by several orders of magnitude with a moderate cost of a few correction laser beams. }, doi = {10.1103/PhysRevA.88.052325}, url = {http://arxiv.org/abs/1305.2798v3}, author = {Chao Shen and Zhe-Xuan Gong and Luming Duan} } @article {1587, title = {Multilingual Summarization: Dimensionality Reduction and a Step Towards Optimal Term Coverage}, journal = {MultiLing (Workshop on Multilingual Multi-document Summarization)}, year = {2013}, month = {2013/08/09}, pages = {55-63}, abstract = {In this paper we present three term weighting approaches for multi-lingual document summarization and give results on the DUC 2002 data as well as on the 2013 Multilingual Wikipedia feature articles data set. We introduce a new intervalbounded nonnegative matrix factorization. We use this new method, latent semantic analysis (LSA), and latent Dirichlet allocation (LDA) to give three term-weighting methods for multi-document multi-lingual summarization. Results on DUC and TAC data, as well as on the MultiLing 2013 data, demonstrate that these methods are very promising, since they achieve oracle coverage scores in the range of humans for 6 of the 10 test languages. Finally, we present three term weighting approaches for the MultiLing13 single document summarization task on the Wikipedia featured articles. Our submissions signifi- cantly outperformed the baseline in 19 out of 41 languages. }, url = {http://aclweb.org/anthology/W/W13/W13-3108.pdf}, author = {John M. Conroy and Sashka T. Davis and Jeff Kubina and Yi-Kai Liu and Dianne P. O{\textquoteright}Leary and Judith D. Schlesinger} } @article {1485, title = {Prethermalization and dynamical transition in an isolated trapped ion spin chain }, journal = {New Journal of Physics}, volume = {15}, year = {2013}, month = {2013/11/26}, pages = {113051}, abstract = { We propose an experimental scheme to observe prethermalization and dynamical transition in one-dimensional XY spin chain with long range interaction and inhomogeneous lattice spacing, which can be readily implemented with the recently developed trapped-ion quantum simulator. Local physical observables are found to relax to prethermal values at intermediate time scale, followed by complete relaxation to thermal values at much longer time. The physical origin of prethermalization is explained by spotting a non-trivial structure in lower half of the energy spectrum. The dynamical behavior of the system is shown to cross difference phases when the interaction range is continuously tuned, indicating the existence of dynamical phase transition. }, doi = {10.1088/1367-2630/15/11/113051}, url = {http://arxiv.org/abs/1305.0985v1}, author = {Zhe-Xuan Gong and L. -M. Duan} } @article {1199, title = {Quantum Logic between Remote Quantum Registers}, journal = {Physical Review A}, volume = {87}, year = {2013}, month = {2013/2/6}, abstract = { We analyze two approaches to quantum state transfer in solid-state spin systems. First, we consider unpolarized spin-chains and extend previous analysis to various experimentally relevant imperfections, including quenched disorder, dynamical decoherence, and uncompensated long range coupling. In finite-length chains, the interplay between disorder-induced localization and decoherence yields a natural optimal channel fidelity, which we calculate. Long-range dipolar couplings induce a finite intrinsic lifetime for the mediating eigenmode; extensive numerical simulations of dipolar chains of lengths up to L=12 show remarkably high fidelity despite these decay processes. We further consider the extension of the protocol to bosonic systems of coupled oscillators. Second, we introduce a quantum mirror based architecture for universal quantum computing which exploits all of the spins in the system as potential qubits. While this dramatically increases the number of qubits available, the composite operations required to manipulate "dark" spin qubits significantly raise the error threshold for robust operation. Finally, as an example, we demonstrate that eigenmode-mediated state transfer can enable robust long-range logic between spatially separated Nitrogen-Vacancy registers in diamond; numerical simulations confirm that high fidelity gates are achievable even in the presence of moderate disorder. }, doi = {10.1103/PhysRevA.87.022306}, url = {http://arxiv.org/abs/1206.0014v1}, author = {Norman Y. Yao and Zhe-Xuan Gong and Chris R. Laumann and Steven D. Bennett and L. -M. Duan and Mikhail D. Lukin and Liang Jiang and Alexey V. Gorshkov} } @article {1345, title = {Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit}, journal = {Nature Nanotechnology}, volume = {8}, year = {2013}, month = {2013/9/1}, pages = {654 - 659}, abstract = {We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes. }, doi = {10.1038/nnano.2013.168}, url = {http://arxiv.org/abs/1302.1933v1}, author = {J. Medford and J. Beil and J. M. Taylor and S. D. Bartlett and A. C. Doherty and E. I. Rashba and D. P. DiVincenzo and H. Lu and A. C. Gossard and C. M. Marcus} } @article {1504, title = {Single-photon nonlinear optics with graphene plasmons}, journal = {Physical Review Letters}, volume = {111}, year = {2013}, month = {2013/12/11}, abstract = { We show that it is possible to realize significant nonlinear optical interactions at the few photon level in graphene nanostructures. Our approach takes advantage of the electric field enhancement associated with the strong confinement of graphene plasmons and the large intrinsic nonlinearity of graphene. Such a system could provide a powerful platform for quantum nonlinear optical control of light. As an example, we consider an integrated optical device that exploits this large nonlinearity to realize a single photon switch. }, doi = {10.1103/PhysRevLett.111.247401}, url = {http://arxiv.org/abs/1309.2651v3}, author = {Michael Gullans and D. E. Chang and F. H. L. Koppens and F. J. Garc{\'\i}a de Abajo and M. D. Lukin} } @article {1459, title = {Uniqueness of Quantum States Compatible with Given Measurement Results}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/7/11}, abstract = { We discuss the uniqueness of quantum states compatible with given results for measuring a set of observables. For a given pure state, we consider two different types of uniqueness: (1) no other pure state is compatible with the same measurement results and (2) no other state, pure or mixed, is compatible with the same measurement results. For case (1), it is known that for a d-dimensional Hilbert space, there exists a set of 4d-5 observables that uniquely determines any pure state. We show that for case (2), 5d-7 observables suffice to uniquely determine any pure state. Thus there is a gap between the results for (1) and (2), and we give some examples to illustrate this. The case of observables corresponding to reduced density matrices (RDMs) of a multipartite system is also discussed, where we improve known bounds on local dimensions for case (2) in which almost all pure states are uniquely determined by their RDMs. We further discuss circumstances where (1) can imply (2). We use convexity of the numerical range of operators to show that when only two observables are measured, (1) always implies (2). More generally, if there is a compact group of symmetries of the state space which has the span of the observables measured as the set of fixed points, then (1) implies (2). We analyze the possible dimensions for the span of such observables. Our results extend naturally to the case of low rank quantum states. }, doi = {10.1103/PhysRevA.88.012109}, url = {http://arxiv.org/abs/1212.3503v2}, author = {Jianxin Chen and Hillary Dawkins and Zhengfeng Ji and Nathaniel Johnston and David Kribs and Frederic Shultz and Bei Zeng} } @article {grant2012galilean, title = {On Galilean connections and the first jet bundle}, journal = {Central European Journal of Mathematics}, volume = {10}, number = {5}, year = {2012}, month = {2012/10/01}, pages = {1889{\textendash}1895}, publisher = {Springer}, abstract = {We see how the first jet bundle of curves into affine space can be realized as a homogeneous space of the Galilean group. Cartan connections with this model are precisely the geometric structure of second-order ordinary differential equations under time-preserving transformations {\textemdash} sometimes called KCC-theory. With certain regularity conditions, we show that any such Cartan connection induces {\textquotedblleft}laboratory{\textquotedblright} coordinate systems, and the geodesic equations in this coordinates form a system of second-order ordinary differential equations. We then show the converse {\textemdash} the {\textquotedblleft}fundamental theorem{\textquotedblright} {\textemdash} that given such a coordinate system, and a system of second order ordinary differential equations, there exists regular Cartan connections yielding these, and such connections are completely determined by their torsion.}, author = {Grant, James DE and Brad Lackey} } @article {1493, title = {Photonic quantum simulation of ground state configurations of Heisenberg square and checkerboard lattice spin systems }, year = {2012}, month = {2012/05/12}, abstract = { Photonic quantum simulators are promising candidates for providing insight into other small- to medium-sized quantum systems. The available photonic quantum technology is reaching the state where significant advantages arise for the quantum simulation of interesting questions in Heisenberg spin systems. Here we experimentally simulate such spin systems with single photons and linear optics. The effective Heisenberg-type interactions among individual single photons are realized by quantum interference at the tunable direction coupler followed by the measurement process. The effective interactions are characterized by comparing the entanglement dynamics using pairwise concurrence of a four-photon quantum system. We further show that photonic quantum simulations of generalized Heisenberg interactions on a four-site square lattice and a six-site checkerboard lattice are in reach of current technology. }, url = {http://arxiv.org/abs/1205.2801v1}, author = {Xiao-song Ma and Borivoje Dakic and Sebastian Kropatsche and William Naylor and Yang-hao Chan and Zhe-Xuan Gong and Lu-ming Duan and Anton Zeilinger and Philip Walther} } @article {1492, title = {Quantum Simulation of Spin Models on an Arbitrary Lattice with Trapped Ions }, journal = {New Journal of Physics}, volume = {14}, year = {2012}, month = {2012/09/27}, pages = {095024}, abstract = { A collection of trapped atomic ions represents one of the most attractive platforms for the quantum simulation of interacting spin networks and quantum magnetism. Spin-dependent optical dipole forces applied to an ion crystal create long-range effective spin-spin interactions and allow the simulation of spin Hamiltonians that possess nontrivial phases and dynamics. Here we show how appropriate design of laser fields can provide for arbitrary multidimensional spin-spin interaction graphs even for the case of a linear spatial array of ions. This scheme uses currently existing trap technology and is scalable to levels where classical methods of simulation are intractable. }, doi = {10.1088/1367-2630/14/9/095024}, url = {http://arxiv.org/abs/1201.0776v1}, author = {Simcha Korenblit and Dvir Kafri and Wess C. Campbell and Rajibul Islam and Emily E. Edwards and Zhe-Xuan Gong and Guin-Dar Lin and Luming Duan and Jungsang Kim and Kihwan Kim and Christopher Monroe} } @article {1491, title = {Reply to Comment on "Space-Time Crystals of Trapped Ions}, year = {2012}, month = {2012/10/15}, abstract = { This is a reply to the comment from Patrick Bruno (arXiv:1211.4792) on our paper (Phys. Rev. Lett. 109, 163001 (2012)). }, doi = {http://dx.doi.org/10.1103/PhysRevLett.109.163001}, url = {http://arxiv.org/abs/1212.6959v2}, author = {Tongcang Li and Zhe-Xuan Gong and Zhang-qi Yin and H. T. Quan and Xiaobo Yin and Peng Zhang and L. -M. Duan and Xiang Zhang} } @article {1794, title = {Space-Time Crystals of Trapped Ions}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/10/19}, pages = {163001}, abstract = {Spontaneous symmetry breaking can lead to the formation of time crystals, as well as spatial crystals. Here we propose a space-time crystal of trapped ions and a method to realize it experimentally by confining ions in a ring-shaped trapping potential with a static magnetic field. The ions spontaneously form a spatial ring crystal due to Coulomb repulsion. This ion crystal can rotate persistently at the lowest quantum energy state in magnetic fields with fractional fluxes. The persistent rotation of trapped ions produces the temporal order, leading to the formation of a space-time crystal. We show that these space-time crystals are robust for direct experimental observation. We also study the effects of finite temperatures on the persistent rotation. The proposed space-time crystals of trapped ions provide a new dimension for exploring many-body physics and emerging properties of matter.}, doi = {10.1103/PhysRevLett.109.163001}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.109.163001}, author = {Tongcang Li and Gong, Zhe-Xuan and Yin, Zhang-Qi and Quan, H. T. and Yin, Xiaobo and Zhang, Peng and Duan, L.-M. and Zhang, Xiang} } @article {1470, title = {Steady-state many-body entanglement of hot reactive fermions}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/12/4}, abstract = { Entanglement is typically created via systematic intervention in the time evolution of an initially unentangled state, which can be achieved by coherent control, carefully tailored non-demolition measurements, or dissipation in the presence of properly engineered reservoirs. In this paper we show that two-component Fermi gases at ~\mu K temperatures naturally evolve, in the presence of reactive two-body collisions, into states with highly entangled (Dicke-type) spin wavefunctions. The entanglement is a steady-state property that emerges---without any intervention---from uncorrelated initial states, and could be used to improve the accuracy of spectroscopy in experiments with fermionic alkaline earth atoms or fermionic groundstate molecules. }, doi = {10.1103/PhysRevLett.109.230501}, url = {http://arxiv.org/abs/1207.4741v1}, author = {Michael Foss-Feig and Andrew J. Daley and James K. Thompson and Ana Maria Rey} } @article {1201, title = {Topological Flat Bands from Dipolar Spin Systems}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/12/26}, abstract = { We propose and analyze a physical system that naturally admits two-dimensional topological nearly flat bands. Our approach utilizes an array of three-level dipoles (effective S = 1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hardcore bosons, namely, the dressed spin-flips. These gauge fields result in topological band structures, whose bandgap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at half-filling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultra-cold polar molecules or spins in the solid state is considered. }, doi = {10.1103/PhysRevLett.109.266804}, url = {http://arxiv.org/abs/1207.4479v3}, author = {Norman Y. Yao and Chris R. Laumann and Alexey V. Gorshkov and Steven D. Bennett and Eugene Demler and Peter Zoller and Mikhail D. Lukin} } @article {1484, title = {Comment on "Foundation of Statistical Mechanics under Experimentally Realistic Conditions" }, year = {2011}, month = {2011/09/22}, abstract = { Reimann [Phys. Rev. Lett. 101, 190403 (2008)] claimed that generic isolated macroscopic quantum system will equilibrate under experimentally realistic conditions by proving a theorem. We here show that the proof is invalid for most many-body systems and is unable to demonstrate equilibration in realistic experiment. }, url = {http://arxiv.org/abs/1109.4696v1}, author = {Zhe-Xuan Gong and L. -M. Duan} } @article {1303, title = {Detecting paired and counterflow superfluidity via dipole oscillations}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/10/27}, abstract = { We suggest an experimentally feasible procedure to observe paired and counterflow superfluidity in ultra-cold atom systems. We study the time evolution of one-dimensional mixtures of bosonic atoms in an optical lattice following an abrupt displacement of an additional weak confining potential. We find that the dynamic responses of the paired superfluid phase for attractive inter-species interactions and the counterflow superfluid phase for repulsive interactions are qualitatively distinct and reflect the quasi long-range order that characterizes these states. These findings suggest a clear experimental procedure to detect these phases, and give an intuitive insight into their dynamics. }, doi = {10.1103/PhysRevA.84.041609}, url = {http://arxiv.org/abs/1103.3513v3}, author = {Anzi Hu and L. Mathey and Eite Tiesinga and Ippei Danshita and Carl J. Williams and Charles W. Clark} } @article {1488, title = {Dynamics of Overhauser Field under nuclear spin diffusion in a quantum dot }, journal = {New Journal of Physics}, volume = {13}, year = {2011}, month = {2011/03/25}, pages = {033036}, abstract = { The coherence of electron spin can be significantly enhanced by locking the Overhauser field from nuclear spins using the nuclear spin preparation. We propose a theoretical model to calculate the long time dynamics of the Overhauser field under intrinsic nuclear spin diffusion in a quantum dot. We obtain a simplified diffusion equation that can be numerically solved and show quantitatively how the Knight shift and the electron-mediated nuclear spin flip-flop affect the nuclear spin diffusion. The results explain several recent experimental observations, where the decay time of Overhauser field is measured under different configurations, including variation of the external magnetic field, the electron spin configuration in a double dot, and the initial nuclear spin polarization rate. }, doi = {10.1088/1367-2630/13/3/033036}, url = {http://arxiv.org/abs/0912.4322v1}, author = {Zhe-Xuan Gong and Zhang-qi Yin and L. -M. Duan} } @article {1447, title = {No-go Theorem for One-way Quantum Computing on Naturally Occurring Two-level Systems }, journal = {Physical Review A}, volume = {83}, year = {2011}, month = {2011/5/9}, abstract = { One-way quantum computing achieves the full power of quantum computation by performing single particle measurements on some many-body entangled state, known as the resource state. As single particle measurements are relatively easy to implement, the preparation of the resource state becomes a crucial task. An appealing approach is simply to cool a strongly correlated quantum many-body system to its ground state. In addition to requiring the ground state of the system to be universal for one-way quantum computing, we also want the Hamiltonian to have non-degenerate ground state protected by a fixed energy gap, to involve only two-body interactions, and to be frustration-free so that measurements in the course of the computation leave the remaining particles in the ground space. Recently, significant efforts have been made to the search of resource states that appear naturally as ground states in spin lattice systems. The approach is proved to be successful in spin-5/2 and spin-3/2 systems. Yet, it remains an open question whether there could be such a natural resource state in a spin-1/2, i.e., qubit system. Here, we give a negative answer to this question by proving that it is impossible for a genuinely entangled qubit states to be a non-degenerate ground state of any two-body frustration-free Hamiltonian. What is more, we prove that every spin-1/2 frustration-free Hamiltonian with two-body interaction always has a ground state that is a product of single- or two-qubit states, a stronger result that is interesting independent of the context of one-way quantum computing. }, doi = {10.1103/PhysRevA.83.050301}, url = {http://arxiv.org/abs/1004.3787v1}, author = {Jianxin Chen and Xie Chen and Runyao Duan and Zhengfeng Ji and Bei Zeng} } @article {1181, title = {Quantum Magnetism with Polar Alkali Dimers}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/9/15}, abstract = { We show that dipolar interactions between ultracold polar alkali dimers in optical lattices can be used to realize a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. The model features long-range spin-spin interactions J_z and J_perp of XXZ type, long-range density-density interaction V, and long-range density-spin interaction W, all of which can be controlled in both magnitude and sign independently of each other and of the tunneling t. The "spin" is encoded in the rotational degree of freedom of the molecules, while the interactions are controlled by applied static electric and continuous-wave microwave fields. Furthermore, we show that nuclear spins of the molecules can be used to implement an additional (orbital) degree of freedom that is coupled to the original rotational degree of freedom in a tunable way. The presented system is expected to exhibit exotic physics and to provide insights into strongly correlated phenomena in condensed matter systems. Realistic experimental imperfections are discussed. }, doi = {10.1103/PhysRevA.84.033619}, url = {http://arxiv.org/abs/1106.1655v1}, author = {Alexey V. Gorshkov and Salvatore R. Manmana and Gang Chen and Eugene Demler and Mikhail D. Lukin and Ana Maria Rey} } @article {1846, title = {Quantum magnetism with polar alkali-metal dimers}, journal = {Phys. Rev. A}, volume = {84}, year = {2011}, pages = {033619}, url = {http://link.aps.org/abstract/PRA/v84/e033619/}, author = {Alexey V. Gorshkov and Manmana, S R and Chen, G and Demler, E and Lukin, M D and Rey, A M} } @article {1196, title = {Robust Quantum State Transfer in Random Unpolarized Spin Chains}, journal = {Physical Review Letters}, volume = {106}, year = {2011}, month = {2011/1/27}, abstract = { We propose and analyze a new approach for quantum state transfer between remote spin qubits. Specifically, we demonstrate that coherent quantum coupling between remote qubits can be achieved via certain classes of random, unpolarized (infinite temperature) spin chains. Our method is robust to coupling strength disorder and does not require manipulation or control over individual spins. In principle, it can be used to attain perfect state transfer over arbitrarily long range via purely Hamiltonian evolution and may be particularly applicable in a solid-state quantum information processor. As an example, we demonstrate that it can be used to attain strong coherent coupling between Nitrogen-Vacancy centers separated by micrometer distances at room temperature. Realistic imperfections and decoherence effects are analyzed. }, doi = {10.1103/PhysRevLett.106.040505}, url = {http://arxiv.org/abs/1011.2762v2}, author = {Norman Y. Yao and Liang Jiang and Alexey V. Gorshkov and Zhe-Xuan Gong and Alex Zhai and L. -M. Duan and Mikhail D. Lukin} } @article {1198, title = {Tunable Superfluidity and Quantum Magnetism with Ultracold Polar Molecules }, journal = {Physical Review Letters}, volume = {107}, year = {2011}, month = {2011/9/8}, abstract = { By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and novel density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method (DMRG) to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally. }, doi = {10.1103/PhysRevLett.107.115301}, url = {http://arxiv.org/abs/1106.1644v1}, author = {Alexey V. Gorshkov and Salvatore R. Manmana and Gang Chen and Jun Ye and Eugene Demler and Mikhail D. Lukin and Ana Maria Rey} } @article {1501, title = {Adiabatic preparation of many-body states in optical lattices}, journal = {Physical Review A}, volume = {81}, year = {2010}, month = {2010/6/22}, abstract = { We analyze a technique for the preparation of low entropy many body states of atoms in optical lattices based on adiabatic passage. In particular, we show that this method allows preparation of strongly correlated states as stable highest energy states of Hamiltonians that have trivial ground states. As an example, we analyze the generation of antiferromagnetically ordered states by adiabatic change of a staggered field acting on the spins of bosonic atoms with ferromagnetic interactions. }, doi = {10.1103/PhysRevA.81.061603}, url = {http://arxiv.org/abs/0906.2567v3}, author = {Anders S. Sorensen and Ehud Altman and Michael Gullans and J. V. Porto and Mikhail D. Lukin and Eugene Demler} } @article {1442, title = {Optimal Perfect Distinguishability between Unitaries and Quantum Operations }, year = {2010}, month = {2010/10/12}, abstract = { We study optimal perfect distinguishability between a unitary and a general quantum operation. In 2-dimensional case we provide a simple sufficient and necessary condition for sequential perfect distinguishability and an analytical formula of optimal query time. We extend the sequential condition to general d-dimensional case. Meanwhile, we provide an upper bound and a lower bound for optimal sequential query time. In the process a new iterative method is given, the most notable innovation of which is its independence to auxiliary systems or entanglement. Following the idea, we further obtain an upper bound and a lower bound of (entanglement-assisted) q-maximal fidelities between a unitary and a quantum operation. Thus by the recursion in [1] an upper bound and a lower bound for optimal general perfect discrimination are achieved. Finally our lower bound result can be extended to the case of arbitrary two quantum operations. }, url = {http://arxiv.org/abs/1010.2298v1}, author = {Cheng Lu and Jianxin Chen and Runyao Duan} } @article {1170, title = {Photonic Phase Gate via an Exchange of Fermionic Spin Waves in a Spin Chain }, journal = {Physical Review Letters}, volume = {105}, year = {2010}, month = {2010/8/5}, abstract = { We propose a new protocol for implementing the two-qubit photonic phase gate. In our approach, the pi phase is acquired by mapping two single photons into atomic excitations with fermionic character and exchanging their positions. The fermionic excitations are realized as spin waves in a spin chain, while photon storage techniques provide the interface between the photons and the spin waves. Possible imperfections and experimental systems suitable for implementing the gate are discussed. }, doi = {10.1103/PhysRevLett.105.060502}, url = {http://arxiv.org/abs/1001.0968v3}, author = {Alexey V. Gorshkov and Johannes Otterbach and Eugene Demler and Michael Fleischhauer and Mikhail D. Lukin} } @article {1489, title = {Temperature driven structural phase transition for trapped ions and its experimental detection }, journal = {Physical Review Letters}, volume = {105}, year = {2010}, month = {2010/12/29}, abstract = { A Wigner crystal formed with trapped ion can undergo structural phase transition, which is determined only by the mechanical conditions on a classical level. Instead of this classical result, we show that through consideration of quantum and thermal fluctuation, a structural phase transition can be solely driven by change of the system{\textquoteright}s temperature. We determine a finite-temperature phase diagram for trapped ions using the renormalization group method and the path integral formalism, and propose an experimental scheme to observe the predicted temperature-driven structural phase transition, which is well within the reach of the current ion trap technology. }, doi = {10.1103/PhysRevLett.105.265703}, url = {http://arxiv.org/abs/1009.0089v1}, author = {Zhe-Xuan Gong and G. -D. Lin and L. -M. Duan} } @article {1850, title = {Two-orbital SU(N) magnetism with ultracold alkaline-earth atoms}, journal = {Nature Phys.}, volume = {6}, year = {2010}, pages = {289}, url = {http://www.nature.com/nphys/journal/v6/n4/abs/nphys1535.html}, author = {Alexey V. Gorshkov and Hermele, M and Gurarie, V and Xu, C and Julienne, P S and Ye, J and Zoller, P and Demler, E and Lukin, M D and Rey, A M} } @article {1193, title = {Alkaline-Earth-Metal Atoms as Few-Qubit Quantum Registers}, journal = {Physical Review Letters}, volume = {102}, year = {2009}, month = {2009/3/18}, abstract = { We propose and analyze a novel approach to quantum information processing, in which multiple qubits can be encoded and manipulated using electronic and nuclear degrees of freedom associated with individual alkaline-earth atoms trapped in an optical lattice. Specifically, we describe how the qubits within each register can be individually manipulated and measured with sub-wavelength optical resolution. We also show how such few-qubit registers can be coupled to each other in optical superlattices via conditional tunneling to form a scalable quantum network. Finally, potential applications to quantum computation and precision measurements are discussed. }, doi = {10.1103/PhysRevLett.102.110503}, url = {http://arxiv.org/abs/0812.3660v2}, author = {Alexey V. Gorshkov and Ana Maria Rey and Andrew J. Daley and Martin M. Boyd and Jun Ye and Peter Zoller and Mikhail D. Lukin} } @article {1299, title = {Counterflow and paired superfluidity in one-dimensional Bose mixtures in optical lattices }, journal = {Physical Review A}, volume = {80}, year = {2009}, month = {2009/8/24}, abstract = { We study the quantum phases of mixtures of ultra-cold bosonic atoms held in an optical lattice that confines motion or hopping to one spatial dimension. The phases are found by using Tomonaga-Luttinger liquid theory as well as the numerical method of time evolving block decimation (TEBD). We consider a binary mixture with repulsive intra-species interactions, and either repulsive or attractive inter-species interaction. For a homogeneous system, we find paired- and counterflow-superfluid phases at different filling and hopping energies. We also predict parameter regions in which these types of superfluid order coexist with charge density wave order. We show that the Tomonaga-Luttinger liquid theory and TEBD qualitatively agree on the location of the phase boundary to superfluidity. We then describe how these phases are modified and can be detected when an additional harmonic trap is present. In particular, we show how experimentally measurable quantities, such as time-of-flight images and the structure factor, can be used to distinguish the quantum phases. Finally, we suggest applying a Feshbach ramp to detect the paired superfluid state, and a $\pi/2$ pulse followed by Bragg spectroscopy to detect the counterflow superfluid phase. }, doi = {10.1103/PhysRevA.80.023619}, url = {http://arxiv.org/abs/0906.2150v1}, author = {Anzi Hu and L. Mathey and Ippei Danshita and Eite Tiesinga and Carl J. Williams and Charles W. Clark} } @article {1195, title = {Realization of Coherent Optically Dense Media via Buffer-Gas Cooling}, journal = {Physical Review A}, volume = {79}, year = {2009}, month = {2009/1/6}, abstract = { We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool 87Rb atoms to below 7 K and slow atom diffusion to the walls. Electromagnetically induced transparency (EIT) in this medium allows for 50\% transmission in a medium with initial OD >70 and for slow pulse propagation with large delay-bandwidth products. In the high-OD regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing. }, doi = {10.1103/PhysRevA.79.013806}, url = {http://arxiv.org/abs/0805.1416v2}, author = {Tao Hong and Alexey V. Gorshkov and David Patterson and Alexander S. Zibrov and John M. Doyle and Mikhail D. Lukin and Mara G. Prentiss} } @article {1192, title = {Anyonic interferometry and protected memories in atomic spin lattices}, journal = {Nature Physics}, volume = {4}, year = {2008}, month = {2008/4/20}, pages = {482 - 488}, abstract = { Strongly correlated quantum systems can exhibit exotic behavior called topological order which is characterized by non-local correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasi-particles with anyonic statistics and have been proposed as candidates for naturally fault-tolerant quantum computation. Despite these remarkable properties, anyons have never been observed in nature directly. Here we describe how to unambiguously detect and characterize such states in recently proposed spin lattice realizations using ultra-cold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by performing global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations. }, doi = {10.1038/nphys943}, url = {http://arxiv.org/abs/0711.1365v1}, author = {Liang Jiang and Gavin K. Brennen and Alexey V. Gorshkov and Klemens Hammerer and Mohammad Hafezi and Eugene Demler and Mikhail D. Lukin and Peter Zoller} } @article {1355, title = {Coherence of an optically illuminated single nuclear spin qubit}, journal = {Physical Review Letters}, volume = {100}, year = {2008}, month = {2008/2/19}, abstract = {We investigate the coherence properties of individual nuclear spin quantum bits in diamond [Dutt et al., Science, 316, 1312 (2007)] when a proximal electronic spin associated with a nitrogen-vacancy (NV) center is being interrogated by optical radiation. The resulting nuclear spin dynamics are governed by time-dependent hyperfine interaction associated with rapid electronic transitions, which can be described by a spin-fluctuator model. We show that due to a process analogous to motional averaging in nuclear magnetic resonance, the nuclear spin coherence can be preserved after a large number of optical excitation cycles. Our theoretical analysis is in good agreement with experimental results. It indicates a novel approach that could potentially isolate the nuclear spin system completely from the electronic environment. }, doi = {10.1103/PhysRevLett.100.073001}, url = {http://arxiv.org/abs/0707.1341v2}, author = {Liang Jiang and M. V. Gurudev Dutt and Emre Togan and Lily Childress and Paola Cappellaro and J. M. Taylor and Mikhail D. Lukin} } @article {1453, title = {Existence of Universal Entangler}, journal = {Journal of Mathematical Physics}, volume = {49}, year = {2008}, month = {2008/01/01}, pages = {012103}, abstract = { A gate is called entangler if it transforms some (pure) product states to entangled states. A universal entangler is a gate which transforms all product states to entangled states. In practice, a universal entangler is a very powerful device for generating entanglements, and thus provides important physical resources for accomplishing many tasks in quantum computing and quantum information. This Letter demonstrates that a universal entangler always exists except for a degenerated case. Nevertheless, the problem how to find a universal entangler remains open. }, doi = {10.1063/1.2829895}, url = {http://arxiv.org/abs/0704.1473v2}, author = {Jianxin Chen and Runyao Duan and Zhengfeng Ji and Mingsheng Ying and Jun Yu} } @article {1305, title = {Multilevel effects in the Rabi oscillations of a Josephson phase qubit}, journal = {Physical Review B}, volume = {78}, year = {2008}, month = {2008/9/15}, abstract = { We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting quantum interference device (SQUID) phase qubit with a 100 um^2 area junction acquired over a range of microwave drive power and frequency detuning. Given the slightly anharmonic level structure of the device, several excited states play an important role in the qubit dynamics, particularly at high power. To investigate the effects of these levels, multiphoton Rabi oscillations were monitored by measuring the tunneling escape rate of the device to the voltage state, which is particularly sensitive to excited state population. We compare the observed oscillation frequencies with a simplified model constructed from the full phase qubit Hamiltonian and also compare time-dependent escape rate measurements with a more complete density-matrix simulation. Good quantitative agreement is found between the data and simulations, allowing us to identify a shift in resonance (analogous to the ac Stark effect), a suppression of the Rabi frequency, and leakage to the higher excited states. }, doi = {10.1103/PhysRevB.78.104510}, url = {http://arxiv.org/abs/0806.4711v2}, author = {S. K. Dutta and Frederick W. Strauch and R. M. Lewis and Kaushik Mitra and Hanhee Paik and T. A. Palomaki and Eite Tiesinga and J. R. Anderson and Alex J. Dragt and C. J. Lobb and F. C. Wellstood} } @article {1465, title = {The Power of Unentanglement}, year = {2008}, month = {2008/04/04}, abstract = { The class QMA(k), introduced by Kobayashi et al., consists of all languages that can be verified using k unentangled quantum proofs. Many of the simplest questions about this class have remained embarrassingly open: for example, can we give any evidence that k quantum proofs are more powerful than one? Does QMA(k)=QMA(2) for k>=2? Can QMA(k) protocols be amplified to exponentially small error? In this paper, we make progress on all of the above questions. First, we give a protocol by which a verifier can be convinced that a 3SAT formula of size n is satisfiable, with constant soundness, given ~O(sqrt(n)) unentangled quantum witnesses with O(log n) qubits each. Our protocol relies on the existence of very short PCPs. Second, we show that assuming a weak version of the Additivity Conjecture from quantum information theory, any QMA(2) protocol can be amplified to exponentially small error, and QMA(k)=QMA(2) for all k>=2. Third, we prove the nonexistence of "perfect disentanglers" for simulating multiple Merlins with one. }, url = {http://arxiv.org/abs/0804.0802v2}, author = {Scott Aaronson and Salman Beigi and Andrew Drucker and Bill Fefferman and Peter Shor} } @article {1362, title = {Solid-state circuit for spin entanglement generation and purification}, journal = {Physical Review Letters}, volume = {94}, year = {2005}, month = {2005/6/15}, abstract = {We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology. }, doi = {10.1103/PhysRevLett.94.236803}, url = {http://arxiv.org/abs/cond-mat/0503255v2}, author = {J. M. Taylor and W. D{\"u}r and P. Zoller and A. Yacoby and C. M. Marcus and M. D. Lukin} } @article {1289, title = {Ultracold collision properties of metastable alkaline-earth atoms}, journal = {Physical Review Letters}, volume = {90}, year = {2003}, month = {2003/2/13}, abstract = { Ultra-cold collisions of spin-polarized 24Mg,40Ca, and 88Sr in the metastable 3P2 excited state are investigated. We calculate the long-range interaction potentials and estimate the scattering length and the collisional loss rate as a function of magnetic field. The estimates are based on molecular potentials between 3P2 alkaline-earth atoms obtained from ab initio atomic and molecular structure calculations. The scattering lengths show resonance behavior due to the appearance of a molecular bound state in a purely long-range interaction potential and are positive for magnetic fields below 50 mT. A loss-rate model shows that losses should be smallest near zero magnetic field and for fields slightly larger than the resonance field, where the scattering length is also positive. }, doi = {10.1103/PhysRevLett.90.063002}, url = {http://arxiv.org/abs/physics/0210076v1}, author = {Andrei Derevianko and Sergey G. Porsev and Svetlana Kotochigova and Eite Tiesinga and Paul S. Julienne} } @article {1263, title = {Exponential algorithmic speedup by quantum walk}, year = {2002}, month = {2002/09/24}, abstract = { We construct an oracular (i.e., black box) problem that can be solved exponentially faster on a quantum computer than on a classical computer. The quantum algorithm is based on a continuous time quantum walk, and thus employs a different technique from previous quantum algorithms based on quantum Fourier transforms. We show how to implement the quantum walk efficiently in our oracular setting. We then show how this quantum walk can be used to solve our problem by rapidly traversing a graph. Finally, we prove that no classical algorithm can solve this problem with high probability in subexponential time. }, doi = {10.1145/780542.780552}, url = {http://arxiv.org/abs/quant-ph/0209131v2}, author = {Andrew M. Childs and Richard Cleve and Enrico Deotto and Edward Farhi and Sam Gutmann and Daniel A. Spielman} } @article {1262, title = {Quantum search by measurement}, journal = {Physical Review A}, volume = {66}, year = {2002}, month = {2002/9/23}, abstract = { We propose a quantum algorithm for solving combinatorial search problems that uses only a sequence of measurements. The algorithm is similar in spirit to quantum computation by adiabatic evolution, in that the goal is to remain in the ground state of a time-varying Hamiltonian. Indeed, we show that the running times of the two algorithms are closely related. We also show how to achieve the quadratic speedup for Grover{\textquoteright}s unstructured search problem with only two measurements. Finally, we discuss some similarities and differences between the adiabatic and measurement algorithms. }, doi = {10.1103/PhysRevA.66.032314}, url = {http://arxiv.org/abs/quant-ph/0204013v1}, author = {Andrew M. Childs and Enrico Deotto and Edward Farhi and Jeffrey Goldstone and Sam Gutmann and Andrew J. Landahl} } @article {1261, title = {Universal simulation of Hamiltonian dynamics for qudits}, journal = {Physical Review A}, volume = {66}, year = {2002}, month = {2002/8/30}, abstract = { What interactions are sufficient to simulate arbitrary quantum dynamics in a composite quantum system? Dodd et al. (quant-ph/0106064) provided a partial solution to this problem in the form of an efficient algorithm to simulate any desired two-body Hamiltonian evolution using any fixed two-body entangling N-qubit Hamiltonian, and local unitaries. We extend this result to the case where the component systems have D dimensions. As a consequence we explain how universal quantum computation can be performed with any fixed two-body entangling N-qudit Hamiltonian, and local unitaries. }, doi = {10.1103/PhysRevA.66.022317}, url = {http://arxiv.org/abs/quant-ph/0109064v2}, author = {Michael A. Nielsen and Michael J. Bremner and Jennifer L. Dodd and Andrew M. Childs and Christopher M. Dawson} }